Fix moc files not using PCH when cotired. Added the possibility to specify the path to stdafx.cpp to cotire instead of passing it as first argument to add_library and add_executable.

[fkorsa]

This merge request fixes two issues that were showstoppers for me. Please integrate them, or not, as you see fit. Everything included here is just a suggestion.

If you want me to refine these commits to be more consistent with your style, I'll be happy to. In this case just tell me the issues you see.