[patch] Factor common code for C-family and Fortran preprocessing

[FX]

> I think you should leave the define of __GNUG__ in c-cppbuiltin.c (just 
> like the __GFORTRAN__ define is staying in the Fortran front-end code).  
> You don't then need an is_cxx parameter.

Yep, but then you have to parse the BASEVER string twice, which is why I decided against it.
Of course, if you still think it's preferable when taking this into account, I'll post an updated patch.

FX