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

[François-Xavier Coudert]

Patch committed as rev. 127739. Danny, as mingw maintainer, I think
it's your call for 4.2: do you want me to commit the patch to the 4.2
branch? It is a regression, technically, because gfortran-4.2 isn't
usable on mingw unless patched)

FX