[patch] Factor common code for C-family and Fortran preprocessing (3rd try)

[FX]

> The C changes are OK.  There are more macros I would think of as 
> language-independent - __EXCEPTIONS, __USING_SJLJ_EXCEPTIONS__, 
> __GCC_HAVE_DWARF2_CFI_ASM, __SSP_ALL__, __SSP__, _OPENMP, __NO_INLINE__, 
> __SIZEOF_INT128__ at least - but they can be dealt with later.

Thanks for the review, I'll look at these macros (does SSP apply to Fortran, for example?).

To Fortran maintainers: I still needing the Fortran part to be okayed.

FX