MSVC 2013 Compile Fix/Workaround for "static thread_local" vars

Apparently MSVC 2013 has trouble with stuff that's been declared
"static thread_local" (and/or maybe even the "thread_local" keyword).

https://stackoverflow.com/questions/29399494/what-is-the-current-state-of-support-for-thread-local-across-platforms

intern/gawain/src/gwn_vertex_array_id.cpp

@@ -46,7 +46,12 @@ struct Gwn_Context {
 #endif
 };
 
+#if defined(_MSC_VER) && (_MSC_VER == 1800)
+#define thread_local __declspec(thread)
+thread_local Gwn_Context* active_ctx = NULL;
+#else
 static thread_local Gwn_Context* active_ctx = NULL;
+#endif
 
 static void clear_orphans(Gwn_Context* ctx)
 	{