don't link rt when oglrenderer disabled on apple

src/CMakeLists.txt

@@ -130,6 +130,8 @@ if (ENABLE_OGLRENDERER)
 else()
 	if (WIN32)
 		target_link_libraries(core ole32 comctl32 ws2_32)
+	elseif (APPLE)
+		target_link_libraries(core)
 	else()
 		target_link_libraries(core rt)
 	endif()
@@ -137,4 +139,4 @@ endif()
 
 if (ENABLE_JIT_PROFILING)
 	target_link_libraries(core jitprofiling)
-endif()
+endif()