[ccache] preprocessing with -P

[Giulio Eulisse]

> verwirren
> 
> I'm not sure, but If you are removing the path of include path in the
> preprocessed file - the debugger will be confused finding its source files - or
> what do you think?

mmm....true...That could be solved by compiling the original source (and
not the preprocessed one) on a cache-miss, though. This way it would
actually pre-process twice on a cachemiss but it would then be allowed
to share the same cache across different versions of the software being
compiled (provided that the interfaces don't change). I think that in my
case (big amount of sources and small changes across different versions)
it would still be a winner. Maybe it is possible to have it as an
option?

Ciao,
Giulio