Add a test for including system GL headers after epoxy.

test/Makefile.am

@@ -32,7 +32,8 @@ AM_CPPFLAGS = \
 	$(X11_CFLAGS)
 
 TESTS = \
-	glx_public_api
+	glx_public_api \
+	headerguards \
 	$()
 
 check_PROGRAMS = $(TESTS)

test/headerguards.c

@@ -0,0 +1,35 @@
+/*
+ * Copyright © 2013 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#include <epoxy/gl.h>
+#include <epoxy/glx.h>
+
+#include <GL/gl.h>
+#include <GL/glext.h>
+#include <GL/glx.h>
+#include <GL/glxext.h>
+
+int main(int argc, char **argv)
+{
+    return 0;
+}