Provide a means to control compiler, preprocessor, etc executabale

[MatthewHambley]

There is all sorts of default magic built in but we might prefer simply to tell Fab what to use.

[GiorgiaLine]

I was about to make my own issue for this but I think this is related enough: if we're going to have compilers controlled by this much magic, we need more metadata so that compiler wrappers can be associated with base compilers (and therefore the correct configuration files).