compiling m4 on RHEL6 beta

Today, I ran into an issue compiling m4 on RHEL6 beta. I had to apply this patch to make it work or I got errors about S_ISDIR not being defined during linking. Any ideas why this was an issue?

diff --git a/src/path.c b/src/path.c
index 682969d..e5870ae 100644
--- a/src/path.c
+++ b/src/path.c
@@ -22,6 +22,7 @@
 /* Handling of path search of included files via the builtins "include"
    and "sinclude".  */
 
+#include "sys/stat.h"
 #include "m4.h"
 
 struct includes