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+# ==============================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_compute_relative_paths.html
+# ==============================================================================
+#
+# SYNOPSIS
+#
+#   AX_COMPUTE_RELATIVE_PATHS(PATH_LIST)
+#
+# DESCRIPTION
+#
+#   PATH_LIST is a space-separated list of colon-separated triplets of the
+#   form 'FROM:TO:RESULT'. This function iterates over these triplets and
+#   set $RESULT to the relative path from $FROM to $TO. Note that $FROM and
+#   $TO needs to be absolute filenames for this macro to success.
+#
+#   For instance,
+#
+#     first=/usr/local/bin
+#     second=/usr/local/share
+#     AX_COMPUTE_RELATIVE_PATHS([first:second:fs second:first:sf])
+#     # $fs is set to ../share
+#     # $sf is set to ../bin
+#
+#   $FROM and $TO are both eval'ed recursively and normalized, this means
+#   that you can call this macro with autoconf's dirnames like `prefix' or
+#   `datadir'. For example:
+#
+#     AX_COMPUTE_RELATIVE_PATHS([bindir:datadir:bin_to_data])
+#
+#   AX_COMPUTE_RELATIVE_PATHS should also works with DOS filenames.
+#
+#   You may want to use this macro in order to make your package
+#   relocatable. Instead of hardcoding $datadir into your programs just
+#   encode $bin_to_data and try to determine $bindir at run-time.
+#
+#   This macro requires AX_NORMALIZE_PATH and AX_RECURSIVE_EVAL.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Alexandre Duret-Lutz <adl@gnu.org>
+#
+#   This program is free software; you can redistribute it and/or modify it
+#   under the terms of the GNU General Public License as published by the
+#   Free Software Foundation; either version 2 of the License, or (at your
+#   option) any later version.
+#
+#   This program is distributed in the hope that it will be useful, but
+#   WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+#   Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License along
+#   with this program. If not, see <https://www.gnu.org/licenses/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 12
+
+AU_ALIAS([ADL_COMPUTE_RELATIVE_PATHS], [AX_COMPUTE_RELATIVE_PATHS])
+AC_DEFUN([AX_COMPUTE_RELATIVE_PATHS],
+[for _lcl_i in $1; do
+  _lcl_from=\[$]`echo "[$]_lcl_i" | sed 's,:.*$,,'`
+  _lcl_to=\[$]`echo "[$]_lcl_i" | sed 's,^[[^:]]*:,,' | sed 's,:[[^:]]*$,,'`
+  _lcl_result_var=`echo "[$]_lcl_i" | sed 's,^.*:,,'`
+  AX_RECURSIVE_EVAL([[$]_lcl_from], [_lcl_from])
+  AX_RECURSIVE_EVAL([[$]_lcl_to], [_lcl_to])
+  _lcl_notation="$_lcl_from$_lcl_to"
+  AX_NORMALIZE_PATH([_lcl_from],['/'])
+  AX_NORMALIZE_PATH([_lcl_to],['/'])
+  AX_COMPUTE_RELATIVE_PATH([_lcl_from], [_lcl_to], [_lcl_result_tmp])
+  AX_NORMALIZE_PATH([_lcl_result_tmp],["[$]_lcl_notation"])
+  eval $_lcl_result_var='[$]_lcl_result_tmp'
+done])
+
+## Note:
+## *****
+## The following helper macros are too fragile to be used out
+## of AX_COMPUTE_RELATIVE_PATHS (mainly because they assume that
+## paths are normalized), that's why I'm keeping them in the same file.
+## Still, some of them maybe worth to reuse.
+
+dnl AX_COMPUTE_RELATIVE_PATH(FROM, TO, RESULT)
+dnl ===========================================
+dnl Compute the relative path to go from $FROM to $TO and set the value
+dnl of $RESULT to that value.  This function work on raw filenames
+dnl (for instead it will considerate /usr//local and /usr/local as
+dnl two distinct paths), you should really use AX_COMPUTE_RELATIVE_PATHS
+dnl instead to have the paths sanitized automatically.
+dnl
+dnl For instance:
+dnl    first_dir=/somewhere/on/my/disk/bin
+dnl    second_dir=/somewhere/on/another/disk/share
+dnl    AX_COMPUTE_RELATIVE_PATH(first_dir, second_dir, first_to_second)
+dnl will set $first_to_second to '../../../another/disk/share'.
+AC_DEFUN([AX_COMPUTE_RELATIVE_PATH],
+[AX_COMPUTE_COMMON_PATH([$1], [$2], [_lcl_common_prefix])
+AX_COMPUTE_BACK_PATH([$1], [_lcl_common_prefix], [_lcl_first_rel])
+AX_COMPUTE_SUFFIX_PATH([$2], [_lcl_common_prefix], [_lcl_second_suffix])
+$3="[$]_lcl_first_rel[$]_lcl_second_suffix"])
+
+dnl AX_COMPUTE_COMMON_PATH(LEFT, RIGHT, RESULT)
+dnl ============================================
+dnl Compute the common path to $LEFT and $RIGHT and set the result to $RESULT.
+dnl
+dnl For instance:
+dnl    first_path=/somewhere/on/my/disk/bin
+dnl    second_path=/somewhere/on/another/disk/share
+dnl    AX_COMPUTE_COMMON_PATH(first_path, second_path, common_path)
+dnl will set $common_path to '/somewhere/on'.
+AC_DEFUN([AX_COMPUTE_COMMON_PATH],
+[$3=''
+_lcl_second_prefix_match=''
+while test "[$]_lcl_second_prefix_match" != 0; do
+  _lcl_first_prefix=`expr "x[$]$1" : "x\([$]$3/*[[^/]]*\)"`
+  _lcl_second_prefix_match=`expr "x[$]$2" : "x[$]_lcl_first_prefix"`
+  if test "[$]_lcl_second_prefix_match" != 0; then
+    if test "[$]_lcl_first_prefix" != "[$]$3"; then
+      $3="[$]_lcl_first_prefix"
+    else
+      _lcl_second_prefix_match=0
+    fi
+  fi
+done])
+
+dnl AX_COMPUTE_SUFFIX_PATH(PATH, SUBPATH, RESULT)
+dnl ==============================================
+dnl Subtract $SUBPATH from $PATH, and set the resulting suffix
+dnl (or the empty string if $SUBPATH is not a subpath of $PATH)
+dnl to $RESULT.
+dnl
+dnl For instance:
+dnl    first_path=/somewhere/on/my/disk/bin
+dnl    second_path=/somewhere/on
+dnl    AX_COMPUTE_SUFFIX_PATH(first_path, second_path, common_path)
+dnl will set $common_path to '/my/disk/bin'.
+AC_DEFUN([AX_COMPUTE_SUFFIX_PATH],
+[$3=`expr "x[$]$1" : "x[$]$2/*\(.*\)"`])
+
+dnl AX_COMPUTE_BACK_PATH(PATH, SUBPATH, RESULT)
+dnl ============================================
+dnl Compute the relative path to go from $PATH to $SUBPATH, knowing that
+dnl $SUBPATH is a subpath of $PATH (any other words, only repeated '../'
+dnl should be needed to move from $PATH to $SUBPATH) and set the value
+dnl of $RESULT to that value.  If $SUBPATH is not a subpath of PATH,
+dnl set $RESULT to the empty string.
+dnl
+dnl For instance:
+dnl    first_path=/somewhere/on/my/disk/bin
+dnl    second_path=/somewhere/on
+dnl    AX_COMPUTE_BACK_PATH(first_path, second_path, back_path)
+dnl will set $back_path to '../../../'.
+AC_DEFUN([AX_COMPUTE_BACK_PATH],
+[AX_COMPUTE_SUFFIX_PATH([$1], [$2], [_lcl_first_suffix])
+$3=''
+_lcl_tmp='xxx'
+while test "[$]_lcl_tmp" != ''; do
+  _lcl_tmp=`expr "x[$]_lcl_first_suffix" : "x[[^/]]*/*\(.*\)"`
+  if test "[$]_lcl_first_suffix" != ''; then
+     _lcl_first_suffix="[$]_lcl_tmp"
+     $3="../[$]$3"
+  fi
+done])