Compatibility with Visual Studio Build Tools 2019

It is possible to install Visual Studio Build Tools 2019 rather than all of VS, which is a smaller installation; but cerbero is not compatible.

At the top of this file: cerbero/ide/vs/env.py, just append the following to vcvarsalls['vs16']:

r'Microsoft Visual Studio\2019\BuildTools\VC\Auxiliary\Build\vcvarsall.bat'