winpthreads: Separate MSVC and MinGW specific flags and targets - don't pass -D__USE_MINGW_ANSI_STDIO=0 to MSVC; - don't build libgcc/fakelib with MSVC. Signed-off-by: Antonin Décimo <antonin@tarides.com> Signed-off-by: Martin Storsjö <martin@martin.st>