[PATCH, fortran] Interoperability with C int128_t types

[FX]

> And also the ChangeLogs and texi changes.

Oh, I only saw that mail after I sent my review. The doc changes are
fine, but I'd change the wording to make it even more obvious: as
128-bit ISO_C_BINDING integer kinds are not a currently existing
extension, we want to avoid surprising people. Maybe saying it in a
different paragraph:

"In addition to the integer named constants required by the Fortran
2003 standard, GNU Fortran provides as an extension named constants
for the 128-bit integer types supported by the C compiler: C_INT128_T,
... [write here the full list]".

Then, I'd add a new column to the table so that extensions are more
clearly labeled: like, using "Extension" as a column header and
putting an "Ext." for the new symbols.

Regards,
FX

-- 
FX Coudert
http://www.homepages.ucl.ac.uk/~uccafco/