Revived Daniel Pfeifer's work on !984 (closed)

The following generators are supported:

• Ninja
• Makefile
• Visual Studio
• Xcode

I've tested it with building Qt Creator, and got a 22% compilation speed increase with MinGW 8.1, 30% with Clang-cl 8.0, and 37% with Visual C++ 2017!

Fixes #1260 (closed)