Use pkg-config, if available, to locate libxml2 during configure.

If pkg-config is installed and knows about libxml2, use its information
rather than asking xml2-config.  Otherwise proceed as before.  This
patch allows "configure --with-libxml" to succeed on platforms that
have pkg-config but not xml2-config, which is likely to soon become
a typical situation.

The old mechanism can be forced by setting XML2_CONFIG explicitly
(hence, build processes that were already doing so will certainly
not need adjustment).  Also, it's now possible to set XML2_CFLAGS
and XML2_LIBS explicitly to override both programs.

There is a small risk of this breaking existing build processes,
if there are multiple libxml2 installations on the machine and
pkg-config disagrees with xml2-config about which to use.  The
only case where that seems really likely is if a builder has tried
to select a non-default xml2-config by putting it early in his PATH
rather than setting XML2_CONFIG.  Plan to warn against that in the
minor release notes.

Back-patch to v10; before that we had no pkg-config infrastructure,
and it doesn't seem worth adding it for this.

Hugh McMaster and Tom Lane; Peter Eisentraut also made an earlier
attempt at this, from which I lifted most of the docs changes.

Discussion: https://postgr.es/m/CAN9BcdvfUwc9Yx5015bLH2TOiQ-M+t_NADBSPhMF7dZ=pLa_iw@mail.gmail.com

3 files changed, 283 insertions, 161 deletions

diff --git a/configure b/configure
index 1a0aca941b..a7cf71b3f1 100755
--- a/configure
+++ b/configure
@@ -702,8 +702,10 @@ LDFLAGS_EX
 with_zlib
 with_system_tzdata
 with_libxslt
-with_libxml
+XML2_LIBS
+XML2_CFLAGS
 XML2_CONFIG
+with_libxml
 UUID_EXTRA_OBJS
 with_uuid
 with_readline
@@ -719,13 +721,13 @@ with_perl
 with_tcl
 ICU_LIBS
 ICU_CFLAGS
-PKG_CONFIG_LIBDIR
-PKG_CONFIG_PATH
-PKG_CONFIG
 with_icu
 enable_thread_safety
 INCLUDES
 autodepend
+PKG_CONFIG_LIBDIR
+PKG_CONFIG_PATH
+PKG_CONFIG
 TAS
 GCC
 CPP
@@ -887,6 +889,8 @@ PKG_CONFIG_LIBDIR
 ICU_CFLAGS
 ICU_LIBS
 XML2_CONFIG
+XML2_CFLAGS
+XML2_LIBS
 LDFLAGS_EX
 LDFLAGS_SL
 PERL
@@ -1589,6 +1593,8 @@ Some influential environment variables:
   ICU_CFLAGS  C compiler flags for ICU, overriding pkg-config
   ICU_LIBS    linker flags for ICU, overriding pkg-config
   XML2_CONFIG path to xml2-config utility
+  XML2_CFLAGS C compiler flags for XML2, overriding pkg-config
+  XML2_LIBS   linker flags for XML2, overriding pkg-config
   LDFLAGS_EX  extra linker flags for linking executables only
   LDFLAGS_SL  extra linker flags for linking shared libraries only
   PERL        Perl program
@@ -7153,6 +7159,129 @@ else
 fi
 
 
+#
+# Set up pkg_config in case we need it below
+#
+
+
+
+
+
+
+
+if test "x$ac_cv_env_PKG_CONFIG_set" != "xset"; then
+	if test -n "$ac_tool_prefix"; then
+  # Extract the first word of "${ac_tool_prefix}pkg-config", so it can be a program name with args.
+set dummy ${ac_tool_prefix}pkg-config; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_PKG_CONFIG+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  case $PKG_CONFIG in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_PKG_CONFIG="$PKG_CONFIG" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_PKG_CONFIG="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+  ;;
+esac
+fi
+PKG_CONFIG=$ac_cv_path_PKG_CONFIG
+if test -n "$PKG_CONFIG"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PKG_CONFIG" >&5
+$as_echo "$PKG_CONFIG" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+fi
+if test -z "$ac_cv_path_PKG_CONFIG"; then
+  ac_pt_PKG_CONFIG=$PKG_CONFIG
+  # Extract the first word of "pkg-config", so it can be a program name with args.
+set dummy pkg-config; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_path_ac_pt_PKG_CONFIG+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  case $ac_pt_PKG_CONFIG in
+  [\\/]* | ?:[\\/]*)
+  ac_cv_path_ac_pt_PKG_CONFIG="$ac_pt_PKG_CONFIG" # Let the user override the test with a path.
+  ;;
+  *)
+  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_path_ac_pt_PKG_CONFIG="$as_dir/$ac_word$ac_exec_ext"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+  ;;
+esac
+fi
+ac_pt_PKG_CONFIG=$ac_cv_path_ac_pt_PKG_CONFIG
+if test -n "$ac_pt_PKG_CONFIG"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_pt_PKG_CONFIG" >&5
+$as_echo "$ac_pt_PKG_CONFIG" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+  if test "x$ac_pt_PKG_CONFIG" = x; then
+    PKG_CONFIG=""
+  else
+    case $cross_compiling:$ac_tool_warned in
+yes:)
+{ $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: using cross tools not prefixed with host triplet" >&5
+$as_echo "$as_me: WARNING: using cross tools not prefixed with host triplet" >&2;}
+ac_tool_warned=yes ;;
+esac
+    PKG_CONFIG=$ac_pt_PKG_CONFIG
+  fi
+else
+  PKG_CONFIG="$ac_cv_path_PKG_CONFIG"
+fi
+
+fi
+if test -n "$PKG_CONFIG"; then
+	_pkg_min_version=0.9.0
+	{ $as_echo "$as_me:${as_lineno-$LINENO}: checking pkg-config is at least version $_pkg_min_version" >&5
+$as_echo_n "checking pkg-config is at least version $_pkg_min_version... " >&6; }
+	if $PKG_CONFIG --atleast-pkgconfig-version $_pkg_min_version; then
+		{ $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+	else
+		{ $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+		PKG_CONFIG=""
+	fi
+fi
 
 #
 # Automatic dependency tracking
@@ -7321,126 +7450,6 @@ $as_echo "$with_icu" >&6; }
 
 if test "$with_icu" = yes; then
 
-
-
-
-
-
-
-if test "x$ac_cv_env_PKG_CONFIG_set" != "xset"; then
-	if test -n "$ac_tool_prefix"; then
-  # Extract the first word of "${ac_tool_prefix}pkg-config", so it can be a program name with args.
-set dummy ${ac_tool_prefix}pkg-config; ac_word=$2
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
-$as_echo_n "checking for $ac_word... " >&6; }
-if ${ac_cv_path_PKG_CONFIG+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  case $PKG_CONFIG in
-  [\\/]* | ?:[\\/]*)
-  ac_cv_path_PKG_CONFIG="$PKG_CONFIG" # Let the user override the test with a path.
-  ;;
-  *)
-  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
-for as_dir in $PATH
-do
-  IFS=$as_save_IFS
-  test -z "$as_dir" && as_dir=.
-    for ac_exec_ext in '' $ac_executable_extensions; do
-  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
-    ac_cv_path_PKG_CONFIG="$as_dir/$ac_word$ac_exec_ext"
-    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
-    break 2
-  fi
-done
-  done
-IFS=$as_save_IFS
-
-  ;;
-esac
-fi
-PKG_CONFIG=$ac_cv_path_PKG_CONFIG
-if test -n "$PKG_CONFIG"; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PKG_CONFIG" >&5
-$as_echo "$PKG_CONFIG" >&6; }
-else
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }
-fi
-
-
-fi
-if test -z "$ac_cv_path_PKG_CONFIG"; then
-  ac_pt_PKG_CONFIG=$PKG_CONFIG
-  # Extract the first word of "pkg-config", so it can be a program name with args.
-set dummy pkg-config; ac_word=$2
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
-$as_echo_n "checking for $ac_word... " >&6; }
-if ${ac_cv_path_ac_pt_PKG_CONFIG+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  case $ac_pt_PKG_CONFIG in
-  [\\/]* | ?:[\\/]*)
-  ac_cv_path_ac_pt_PKG_CONFIG="$ac_pt_PKG_CONFIG" # Let the user override the test with a path.
-  ;;
-  *)
-  as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
-for as_dir in $PATH
-do
-  IFS=$as_save_IFS
-  test -z "$as_dir" && as_dir=.
-    for ac_exec_ext in '' $ac_executable_extensions; do
-  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
-    ac_cv_path_ac_pt_PKG_CONFIG="$as_dir/$ac_word$ac_exec_ext"
-    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
-    break 2
-  fi
-done
-  done
-IFS=$as_save_IFS
-
-  ;;
-esac
-fi
-ac_pt_PKG_CONFIG=$ac_cv_path_ac_pt_PKG_CONFIG
-if test -n "$ac_pt_PKG_CONFIG"; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_pt_PKG_CONFIG" >&5
-$as_echo "$ac_pt_PKG_CONFIG" >&6; }
-else
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }
-fi
-
-  if test "x$ac_pt_PKG_CONFIG" = x; then
-    PKG_CONFIG=""
-  else
-    case $cross_compiling:$ac_tool_warned in
-yes:)
-{ $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: using cross tools not prefixed with host triplet" >&5
-$as_echo "$as_me: WARNING: using cross tools not prefixed with host triplet" >&2;}
-ac_tool_warned=yes ;;
-esac
-    PKG_CONFIG=$ac_pt_PKG_CONFIG
-  fi
-else
-  PKG_CONFIG="$ac_cv_path_PKG_CONFIG"
-fi
-
-fi
-if test -n "$PKG_CONFIG"; then
-	_pkg_min_version=0.9.0
-	{ $as_echo "$as_me:${as_lineno-$LINENO}: checking pkg-config is at least version $_pkg_min_version" >&5
-$as_echo_n "checking pkg-config is at least version $_pkg_min_version... " >&6; }
-	if $PKG_CONFIG --atleast-pkgconfig-version $_pkg_min_version; then
-		{ $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
-$as_echo "yes" >&6; }
-	else
-		{ $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }
-		PKG_CONFIG=""
-	fi
-fi
-
 pkg_failed=no
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for icu-uc icu-i18n" >&5
 $as_echo_n "checking for icu-uc icu-i18n... " >&6; }
@@ -8133,6 +8142,8 @@ fi
 #
 # XML
 #
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether to build with XML support" >&5
+$as_echo_n "checking whether to build with XML support... " >&6; }
 
 
 
@@ -8159,9 +8170,89 @@ else
 fi
 
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $with_libxml" >&5
+$as_echo "$with_libxml" >&6; }
+
 
 if test "$with_libxml" = yes ; then
-  if test -z "$XML2_CONFIG"; then
+  # Check pkg-config, then xml2-config.  But for backwards compatibility,
+  # setting XML2_CONFIG overrides pkg-config.
+    have_libxml2_pkg_config=no
+  if test -z "$XML2_CONFIG" -a -n "$PKG_CONFIG"; then
+
+pkg_failed=no
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for libxml-2.0 >= 2.6.23" >&5
+$as_echo_n "checking for libxml-2.0 >= 2.6.23... " >&6; }
+
+if test -n "$XML2_CFLAGS"; then
+    pkg_cv_XML2_CFLAGS="$XML2_CFLAGS"
+ elif test -n "$PKG_CONFIG"; then
+    if test -n "$PKG_CONFIG" && \
+    { { $as_echo "$as_me:${as_lineno-$LINENO}: \$PKG_CONFIG --exists --print-errors \"libxml-2.0 >= 2.6.23\""; } >&5
+  ($PKG_CONFIG --exists --print-errors "libxml-2.0 >= 2.6.23") 2>&5
+  ac_status=$?
+  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; then
+  pkg_cv_XML2_CFLAGS=`$PKG_CONFIG --cflags "libxml-2.0 >= 2.6.23" 2>/dev/null`
+		      test "x$?" != "x0" && pkg_failed=yes
+else
+  pkg_failed=yes
+fi
+ else
+    pkg_failed=untried
+fi
+if test -n "$XML2_LIBS"; then
+    pkg_cv_XML2_LIBS="$XML2_LIBS"
+ elif test -n "$PKG_CONFIG"; then
+    if test -n "$PKG_CONFIG" && \
+    { { $as_echo "$as_me:${as_lineno-$LINENO}: \$PKG_CONFIG --exists --print-errors \"libxml-2.0 >= 2.6.23\""; } >&5
+  ($PKG_CONFIG --exists --print-errors "libxml-2.0 >= 2.6.23") 2>&5
+  ac_status=$?
+  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; then
+  pkg_cv_XML2_LIBS=`$PKG_CONFIG --libs "libxml-2.0 >= 2.6.23" 2>/dev/null`
+		      test "x$?" != "x0" && pkg_failed=yes
+else
+  pkg_failed=yes
+fi
+ else
+    pkg_failed=untried
+fi
+
+
+
+if test $pkg_failed = yes; then
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+
+if $PKG_CONFIG --atleast-pkgconfig-version 0.20; then
+        _pkg_short_errors_supported=yes
+else
+        _pkg_short_errors_supported=no
+fi
+        if test $_pkg_short_errors_supported = yes; then
+	        XML2_PKG_ERRORS=`$PKG_CONFIG --short-errors --print-errors --cflags --libs "libxml-2.0 >= 2.6.23" 2>&1`
+        else
+	        XML2_PKG_ERRORS=`$PKG_CONFIG --print-errors --cflags --libs "libxml-2.0 >= 2.6.23" 2>&1`
+        fi
+	# Put the nasty error message in config.log where it belongs
+	echo "$XML2_PKG_ERRORS" >&5
+
+	# do nothing
+elif test $pkg_failed = untried; then
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+	# do nothing
+else
+	XML2_CFLAGS=$pkg_cv_XML2_CFLAGS
+	XML2_LIBS=$pkg_cv_XML2_LIBS
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+	have_libxml2_pkg_config=yes
+fi
+  fi
+  if test "$have_libxml2_pkg_config" = no ; then
+    if test -z "$XML2_CONFIG"; then
   for ac_prog in xml2-config
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
@@ -8216,21 +8307,24 @@ $as_echo "$XML2_CONFIG" >&6; }
 fi
 
     if test -n "$XML2_CONFIG"; then
-    for pgac_option in `$XML2_CONFIG --cflags`; do
-      case $pgac_option in
-        -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
-      esac
-    done
-    for pgac_option in `$XML2_CONFIG --libs`; do
-      case $pgac_option in
-        -L*) LDFLAGS="$LDFLAGS $pgac_option";;
-      esac
-    done
+      XML2_CFLAGS=`$XML2_CONFIG --cflags`
+      XML2_LIBS=`$XML2_CONFIG --libs`
+    fi
   fi
+  # Note the user could also set XML2_CFLAGS/XML2_LIBS directly
+  for pgac_option in $XML2_CFLAGS; do
+    case $pgac_option in
+      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+    esac
+  done
+  for pgac_option in $XML2_LIBS; do
+    case $pgac_option in
+      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+    esac
+  done
 fi
 
 
-
 #
 # XSLT
 #
diff --git a/configure.in b/configure.in
index b27708e336..d36a7e94b3 100644
--- a/configure.in
+++ b/configure.in
@@ -667,6 +667,10 @@ else
 fi
 AC_SUBST(TAS)
 
+#
+# Set up pkg_config in case we need it below
+#
+PKG_PROG_PKG_CONFIG
 
 #
 # Automatic dependency tracking
@@ -927,27 +931,41 @@ AC_SUBST(UUID_EXTRA_OBJS)
 #
 # XML
 #
+AC_MSG_CHECKING([whether to build with XML support])
 PGAC_ARG_BOOL(with, libxml, no, [build with XML support],
               [AC_DEFINE([USE_LIBXML], 1, [Define to 1 to build with XML support. (--with-libxml)])])
+AC_MSG_RESULT([$with_libxml])
+AC_SUBST(with_libxml)
 
 if test "$with_libxml" = yes ; then
-  PGAC_PATH_PROGS(XML2_CONFIG, xml2-config)
+  # Check pkg-config, then xml2-config.  But for backwards compatibility,
+  # setting XML2_CONFIG overrides pkg-config.
   AC_ARG_VAR(XML2_CONFIG, [path to xml2-config utility])dnl
-  if test -n "$XML2_CONFIG"; then
-    for pgac_option in `$XML2_CONFIG --cflags`; do
-      case $pgac_option in
-        -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
-      esac
-    done
-    for pgac_option in `$XML2_CONFIG --libs`; do
-      case $pgac_option in
-        -L*) LDFLAGS="$LDFLAGS $pgac_option";;
-      esac
-    done
+  have_libxml2_pkg_config=no
+  if test -z "$XML2_CONFIG" -a -n "$PKG_CONFIG"; then
+    PKG_CHECK_MODULES(XML2, [libxml-2.0 >= 2.6.23],
+                      [have_libxml2_pkg_config=yes], [# do nothing])
   fi
+  if test "$have_libxml2_pkg_config" = no ; then
+    PGAC_PATH_PROGS(XML2_CONFIG, xml2-config)
+    if test -n "$XML2_CONFIG"; then
+      XML2_CFLAGS=`$XML2_CONFIG --cflags`
+      XML2_LIBS=`$XML2_CONFIG --libs`
+    fi
+  fi
+  # Note the user could also set XML2_CFLAGS/XML2_LIBS directly
+  for pgac_option in $XML2_CFLAGS; do
+    case $pgac_option in
+      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+    esac
+  done
+  for pgac_option in $XML2_LIBS; do
+    case $pgac_option in
+      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+    esac
+  done
 fi
 
-AC_SUBST(with_libxml)
 
 #
 # XSLT
diff --git a/doc/src/sgml/installation.sgml b/doc/src/sgml/installation.sgml
index cc242dccee..4f89b4e930 100644
--- a/doc/src/sgml/installation.sgml
+++ b/doc/src/sgml/installation.sgml
@@ -1117,21 +1117,31 @@ build-postgresql:
        <term><option>--with-libxml</option></term>
        <listitem>
         <para>
-         Build with libxml, enabling SQL/XML support.  Libxml version 2.6.23 or
+         Build with libxml2, enabling SQL/XML support.  Libxml2 version 2.6.23 or
          later is required for this feature.
         </para>
 
         <para>
-         Libxml installs a program <command>xml2-config</command> that
-         can be used to detect the required compiler and linker
-         options.  PostgreSQL will use it automatically if found.  To
-         specify a libxml installation at an unusual location, you can
-         either set the environment variable
-         <envar>XML2_CONFIG</envar> to point to the
-         <command>xml2-config</command> program belonging to the
-         installation, or use the options
-         <option>--with-includes</option> and
-         <option>--with-libraries</option>.
+         To detect the required compiler and linker options, PostgreSQL will
+         query <command>pkg-config</command>, if that is installed and knows
+         about libxml2.  Otherwise the program <command>xml2-config</command>,
+         which is installed by libxml2, will be used if it is found.  Use
+         of <command>pkg-config</command> is preferred, because it can deal
+         with multi-architecture installations better.
+        </para>
+
+        <para>
+         To use a libxml2 installation that is in an unusual location, you
+         can set <command>pkg-config</command>-related environment
+         variables (see its documentation), or set the environment variable
+         <envar>XML2_CONFIG</envar> to point to
+         the <command>xml2-config</command> program belonging to the libxml2
+         installation, or set the variables <envar>XML2_CFLAGS</envar>
+         and <envar>XML2_LIBS</envar>.  (If <command>pkg-config</command> is
+         installed, then to override its idea of where libxml2 is you must
+         either set <envar>XML2_CONFIG</envar> or set
+         both <envar>XML2_CFLAGS</envar> and <envar>XML2_LIBS</envar> to
+         nonempty strings.)
         </para>
        </listitem>
       </varlistentry>
@@ -1857,7 +1867,7 @@ build-postgresql:
        <listitem>
         <para>
          <command>xml2-config</command> program used to locate the
-         libxml installation
+         libxml2 installation
         </para>
        </listitem>
       </varlistentry>