Failure to build on Mojave due to missing wchar.h

I tried to run `pip3 install --upgrade -r requirements.txt` as per instructions on macOS Mojave 10.14.6

While attempting to install `xgboost`, I get the error `wchar.h: No such file or directory`

Looks like this is due to developer tools needing to be reinstalled.

It would be good to detect that these tools need to be installed/upgraded and tell the user, rather than having them read a stack trace and search the web at random for solutions ;)