Require WindowsXP for mingw32.

This allows an i686-pc-mingw32 build to use getaddrinfo/
 freeaddrinfo correctly.
It's not needed for an i686-w64-mingw32 build, but it doesn't hurt
 to leave it in here.
i686-pc-mingw32 still won't work correctly, because it does not
 support fseeko/ftello, nor (unless using MSVCRT 8.0 or later)
 _fseeki64/_ftelli64.
We could define __MSVCRT_VERSION__ to 0x800 to get the latter, but
 this would require different MSVCRT versions than libogg and
 libopus are built with by default, which makes a bit of a mess.