MSYS/MinGW uses winsock but obviously doesn't set _MSC_VER. Use _WIN32
to decide whether to use winsock or BSD headers. Also remove these
headers from src/transport_git.c altogether, as they are not needed.

MSYS is very conservative, so we have to tell it that we don't care
about versions of Windows lower than WindowsXP. We also need to tell
CMake to add ws2_32 to the libraries list and we shouldn't add the
-fPIC option, to MSYS because it complains that it does it anyway.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>