build: prevent autopoint from overwriting a newer M4 file from gnulib

For some reason, 'autopoint' (invoked by 'autoreconf') insists on
overwriting 'extern-inline.m4', even though the version from gnulib
is newer and contains a fix for a build issue on macOS with GCC 11.

So, do as the documentation says, and invoke 'autopoint' before
'gnulib-tool' (and call 'aclocal' to 'automake' afterwards).

  https://www.gnu.org/software/gnulib/manual/html_node/gettextize-and-autopoint.html

This fixes https://savannah.gnu.org/bugs/?61719.
Reported-by: Sam James <sam@cmpct.info>

autogen.sh

@@ -41,9 +41,14 @@ if [ "${gnulib_hash}" != "${curr_hash}" ]; then
 fi
 cd .. >/dev/null || exit 1
 
+autopoint --force
+
 rm -rf lib
 ./gnulib/gnulib-tool \
 	--import \
 	${modules}
 
-autoreconf -f -i -s
+aclocal -I m4
+autoconf
+autoheader
+automake --add-missing