Build fixes for Red Hat 6.2.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@248 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/vg_include.h b/vg_include.h
index 5dca560..f280ef8 100644
--- a/vg_include.h
+++ b/vg_include.h
@@ -672,7 +672,9 @@
    ------------------------------------------------------------------ */
 
 
-#define NULL ((void*)0)
+#if !defined(NULL)
+#  define NULL ((void*)0)
+#endif
 
 extern void VG_(exit)( Int status )
             __attribute__ ((__noreturn__));