configury: bail out early if GNU M4 is not on the path.

configury: bail out early if GNU M4 is not on the path.

Now that libtoolize requires an installed GNU M4 to parse
configure.ac and aclocal.m4 sources for libltdl macros, let the
user know at configure time when it is missing.
* m4/m4.m4: New file for rejecting non-GNU and buggy GNU versions
of M4. Copied from GNU Autoconf m4.m4.
* Makefile.am (lt_aclocal_m4_deps): Add m4/m4.m4.
* configure.ac (AC_PROG_GNU_M4): Call it.
* NEWS: Update.
Reported by Michael Felt

Signed-off-by: Gary V. Vaughan <gary@gnu.org>