[build2] Build system module for code-generation utilities Qt moc, etc

[Patrick Fromberg]

The discussion was about how detecting which files in a Qt project 
require calling MOC on them before compiling.

Such cpp files are hundred percent c++ files from the compiler's point 
of view. However those files are actually not exactly cpp files. They 
have some additional information for moc "secretly" encoded in c++.

I don't think this is a bad thing ( I actually like MOC ) but would it 
not make sense to give those files a different file name ending?

If there is a strong argument for this different file name ending ( not 
just from the point of view of build2 ) then the Qt team may even adopt 
this.