Better checks for M4 in configure.

aclocal.m4

@@ -145,3 +145,16 @@ if test -z "$bird_cv_sys_linux_version" ; then
 	AC_MSG_ERROR([Cannot determine kernel version])
 fi
 ])])
+
+# BIRD_CHECK_PROG_FLAVOR_GNU(PROGRAM-PATH, IF-SUCCESS, [IF-FAILURE])
+# copied autoconf internal _AC_PATH_PROG_FLAVOR_GNU
+m4_define([BIRD_CHECK_PROG_FLAVOR_GNU],
+[# Check for GNU $1
+case `"$1" --version 2>&1` in
+*GNU*)
+  $2;;
+m4_ifval([$3],
+[*)
+  $3;;
+])esac
+])#

configure.in

@@ -14,6 +14,9 @@ AC_ARG_ENABLE(ipv6,[  --enable-ipv6           enable building of IPv6 version (d
 AC_ARG_WITH(sysconfig,[  --with-sysconfig=FILE   use specified BIRD system configuration file])
 AC_ARG_WITH(protocols,[  --with-protocols=LIST   include specified routing protocols (default: all)],,[with_protocols="all"])
 AC_ARG_WITH(sysinclude,[  --with-sysinclude=PATH  search for system includes on specified place])
+AC_ARG_VAR([FLEX], [location of the Flex program]) 
+AC_ARG_VAR([BISON], [location of the Bison program]) 
+AC_ARG_VAR([M4], [location of the M4 program]) 
 
 if test "$srcdir" = . ; then
 	# Building in current directory => create obj directory holding all objects
@@ -76,10 +79,12 @@ AC_PROG_INSTALL
 AC_PROG_RANLIB
 AC_CHECK_PROG(FLEX, flex, flex)
 AC_CHECK_PROG(BISON, bison, bison)
-AC_CHECK_PROGS(M4, gm4 m4, m4)
-if test -z "$FLEX" -o -z "$BISON" -o -z "$M4" ; then
-	AC_MSG_ERROR([Some tools required for building BIRD are missing.])
-	fi
+AC_CHECK_PROGS(M4, gm4 m4)
+
+test -z "$FLEX"	 && AC_MSG_ERROR([Flex is missing.])
+test -z "$BISON" && AC_MSG_ERROR([Bison is missing.])
+test -z "$M4"	 && AC_MSG_ERROR([M4 is missing.])
+BIRD_CHECK_PROG_FLAVOR_GNU([$M4], , [AC_MSG_ERROR([Provided M4 is not GNU M4.])])
 
 if test -n "$with_sysconfig" -a "$with_sysconfig" != no ; then
 	if test -f $with_sysconfig ; then