Explicitly disable inclusion for test and examples

Thank you, Travis CI, for reminding me that one cannot disable a header
with inclusion guards if it doesn't exist.

(cherry picked from commit 537ea4ccf1)

tests/empty.c

@@ -30,6 +30,7 @@
 #include "tinycthread.h"
 
 #include <glad/gl.h>
+#define GLFW_INCLUDE_NONE
 #include <GLFW/glfw3.h>
 
 #include <math.h>