Avoid Apple system libraries when compiling against OSMesa

Rendering/OpenGL/CMakeLists.txt

@@ -175,7 +175,7 @@ if(WIN32)
   if(DirectX_FOUND)
     set(VTK_USE_DIRECTX 1)
   endif()
-elseif(APPLE)
+elseif(APPLE AND NOT VTK_USE_OSMESA)
   set(VTK_USE_APPLE_LOADER 1)
   set(VTK_USE_CORE_GRAPHICS 1)
 else()