tools/gnulib: ignore AC_PREREQ error if actual version passes

The gnulib-tool script is written to have a fatal error
whenever the minimum required version of autoconf
for the project that gnulib is being imported into
as defined in configure.ac was less than
the minimum required version required by gnulib.

However, none of this matters if the version of autoconf
that we use is newer than both requirements.

Instead, use functions from the bootstrap script
to check for the version of autoconf being used
and print a warning whenever this case occurs.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/15690
Signed-off-by: Robert Marko <robimarko@gmail.com>

tools/gnulib/patches/010-autoconf-version.patch

@@ -0,0 +1,47 @@
+--- a/gnulib-tool
++++ b/gnulib-tool
+@@ -346,6 +346,34 @@ Options for --import, --add/remove-impor
+ Report bugs to <bug-gnulib@gnu.org>."
+ }
+ 
++get_version_sed='
++# Move version to start of line.
++s/.*[v ]\([0-9]\)/\1/
++
++# Skip lines that do not start with version.
++/^[0-9]/!d
++
++# Remove characters after the version.
++s/[^.a-z0-9-].*//
++
++# The first component must be digits only.
++s/^\([0-9]*\)[a-z-].*/\1/
++
++# The following essentially does s/5.005/5.5/
++s/\.0*\([1-9]\)/.\1/g
++p
++q'
++
++# get_version
++# copied from build-aux/bootstrap
++get_version () {
++  app=$1
++
++  $app --version >/dev/null 2>&1 || { $app --version; return 1; }
++
++  $app --version 2>&1 | sed -n "$get_version_sed"
++}
++
+ # func_version
+ # outputs to stdout the --version message.
+ func_version ()
+@@ -1620,6 +1648,9 @@ func_determine_path_separator
+   fi
+   case "$autoconf_minversion" in
+     1.* | 2.[0-5]* | 2.6[0-3]*)
++      # if the version of autoconf in use is high enough, do not error
++      case "$(get_version autoconf)" in 1.* | 2.[0-5]* | 2.6[0-3]*) false ;; esac && \
++      func_warning "gnulib requires a newer version of autoconf than configure.ac ( $DEFAULT_AUTOCONF_MINVERSION > AC_PREREQ([$autoconf_minversion]) )" || \
+       func_fatal_error "minimum supported autoconf version is 2.64. Try adding AC_PREREQ([$DEFAULT_AUTOCONF_MINVERSION]) to your configure.ac." ;;
+   esac
+