[configure.ac] Improvements concerning the default CFLAGS with GCC.

  * Add -Wc++-compat option to CFLAGS with GCC ≥ 4.1 not in C++ mode.
    Changes based on Bruno Haible's patch suggested at
      https://sympa.inria.fr/sympa/arc/mpfr/2020-06/msg00008.html
    (with some autoconf related fixes).
  * Output the $CFLAGS value that is eventually used.

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/trunk@14020 280ebfd0-de03-0410-8827-d642c229c3f4

1 files changed, 8 insertions, 2 deletions

diff --git a/configure.ac b/configure.ac
index 38d59c65e..bf7c8835e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -458,8 +458,6 @@ dnl (__GMP_CFLAGS macro in gmp.h) above. The consequence is that this
 dnl might yield a compilation failure if the -Werror option appears in
 dnl __GMP_CFLAGS. But in this case, since -Werror is not used by default,
 dnl one may expect that the user would also set CFLAGS for MPFR.
-dnl Note: When the compiler is GCC not in C++ mode, -Wc++-compat may be
-dnl interesting, but it has been introduced in GCC 4.1 only.
 if test "$test_CFLAGS" != set && test -n "$GCC"; then
   CFLAGS="-Wpointer-arith $CFLAGS"
   AC_MSG_CHECKING(whether the selected language is C++)
@@ -470,12 +468,20 @@ error
 #endif
   ]], [[]])],[
     AC_MSG_RESULT(no)
+    dnl The option -Wc++-compat is available in GCC 4.1.0 and newer.
+    [
+    case `$CC --version | $SED -e 's/^[^ ]* //' -e 's/([^)]*) //' -e 1q` in
+      2.*|3.*|4.0*) ;;
+      *) CFLAGS="-Wc++-compat $CFLAGS" ;;
+    esac
     CFLAGS="-Wmissing-prototypes $CFLAGS"
+    ]
   ],[
     AC_MSG_RESULT(yes)
     CFLAGS="-Wmissing-declarations -Wno-sign-compare $CFLAGS"
   ])
   CFLAGS="-Wall $CFLAGS"
+  AC_MSG_NOTICE([using CFLAGS="$CFLAGS"])
 fi
 
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