[libgfortran,patch] Use __mingw_snprintf instead of snprintf when possible

[François-Xavier Coudert]

>> not regtested, as it requires a very weird setup on mingw
>
> How 'weird'?  Regtested on native build of mingw32 (using cygwin's
> dejagnu support) with no new regressions.

Well, I've never managed to set it up. I do native mingw builds (using
MSYS), and I also have cygwin installed. My previous attempts to use
cygwin's dejagnu to regtest on mingw all failed, and my postings to
the lists never got an answer. And the latest mingw gcc-testresults
posting is for gcc-3.4.3...

So, if you have time to tell in a few words how to get going, I'd
really appreciate it.

Thanks,
FX