Made system header inclusion more conservative.

lib/glext.c

diff --git a/lib/glext.c b/lib/glext.c
index 84b529e..296c7f5 100644
--- a/lib/glext.c
+++ b/lib/glext.c
@@ -30,6 +30,8 @@
 
 #include "internal.h"
 
+#include <string.h>
+
 
 //************************************************************************
 //****                  GLFW internal functions                       ****