m4: add patch to fix build with glibc-2.12.

srcpkgs/m4/patches/missing_S_ISDIR_reference.patch

@@ -0,0 +1,13 @@
+--- src/path.c.orig	2010-05-10 10:53:12.000000000 +0200
++++ src/path.c	2010-05-10 10:54:06.000000000 +0200
+@@ -24,6 +24,10 @@
+ 
+ #include "m4.h"
+ 
++#include <sys/types.h>
++#include <sys/stat.h>
++#include <unistd.h>
++
+ struct includes
+ {
+   struct includes *next;        /* next directory to search */