Replace BISON_PROG_GNU_M4 with Autoconf's AC_PROG_GNU_M4.

If the first m4 in $PATH is wrong, it keeps looking.  Moreover, its
requirements for a correct m4 are stricter.
* m4/m4.m4: Make it a symbolic link to submodules/autoconf/m4/m4.m4.
* configure.ac: Update to use AC_PROG_GNU_M4.
Reported by Eric Blake.

m4/m4.m4

@@ -1,27 +0,0 @@
-# Copyright 2000 Free Software Foundation, Inc.
-#
-# 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 3 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 <http://www.gnu.org/licenses/>.
-
-# BISON_PROG_GNU_M4
-# -----------------
-# Check for GNU m4, at least 1.3 (supports frozen files).
-AC_DEFUN([BISON_PROG_GNU_M4],
-[AC_PATH_PROGS(M4, gm4 gnum4 m4, m4)
-AC_CACHE_CHECK(whether m4 supports frozen files, ac_cv_prog_gnu_m4,
-[ac_cv_prog_gnu_m4=no
-if test x"$M4" != x; then
-  case `$M4 --help < /dev/null 2>&1` in
-    *reload-state*) ac_cv_prog_gnu_m4=yes ;;
-  esac
-fi])])

m4/m4.m4

@@ -0,0 +1 @@
+../submodules/autoconf/m4/m4.m4