Enable multi-threaded mode by default, add warning message if 'Debug' on macOS
Make multi-threaded mode enabled by default
Since I only saw a problem while building in 'Debug' mode on macOS, I now make the multi-threaded mode enabled by default.
I also added a warning if users chose to build in 'Debug' mode on macOS, so they are aware of the problem. Also, in that case, we suggest them to build in 'RelWithDebInfo' instead, whether they need to get debug symbols.