Made Clang & GCC minimum supported version explicit.

src/timer.cpp

@@ -40,7 +40,7 @@ namespace bx
 		int64_t i64 = int64_t(1000.0f * emscripten_get_now() );
 #elif !BX_PLATFORM_NONE
 		struct timeval now;
-		gettimeofday(&now, 0);
+		gettimeofday(&now, NULL);
 		int64_t i64 = now.tv_sec*INT64_C(1000000) + now.tv_usec;
 #else
 		BX_ASSERT(false, "Not implemented!");