Fix glext.h regression in glfwinfo on OS X

The glfwinfo program uses symbols not provided by the glext.h header in
every development environment.

Closes #572.

tests/glfwinfo.c

@@ -23,9 +23,14 @@
 //
 //========================================================================
 
-#define GLFW_INCLUDE_GLEXT
 #include <GLFW/glfw3.h>
 
+/* HACK: Explicitly include the glext.h shipping with GLFW, as this program uses
+ *       many modern symbols not provided by the versions in some development
+ *       environments (for example on OS X).
+ */
+#include <GL/glext.h>
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>