Fixed OSX build.

3rdparty/ocornut-imgui/imgui.cpp

@@ -329,9 +329,7 @@
 #pragma clang diagnostic ignored "-Wexit-time-destructors"  // warning : declaration requires an exit-time destructor       // exit-time destruction order is undefined. if MemFree() leads to users code that has been disabled before exit it might cause problems. ImGui coding style welcomes static/globals.
 #pragma clang diagnostic ignored "-Wglobal-constructors"    // warning : declaration requires a global destructor           // similar to above, not sure what the exact difference it.
 #pragma clang diagnostic ignored "-Wsign-conversion"        // warning : implicit conversion chanjges signedness            //
-#endif
-
-#ifdef __GNUC__
+#elif defined(__GNUC__)
 #pragma GCC diagnostic ignored "-Wtype-limits"
 #pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
 #pragma GCC diagnostic ignored "-Wunused-function"