The forcing of this define was added upstream many years ago, probably
to expose certain features that aren't available by default in the old
mingw.org headers. In mingw-w64, setting __MSVCRT_VERSION__ doesn't
expose much new functionality at all (contrary to in mingw.org), and
the default in mingw-w64 is 0x0700.

Therefore, since we're using mingw-w64 anyway, don't force this version
but just use whatever the toolchain default is, assuming the default
is new enough.

This fixes building with mingw-w64 with the default msvcrt set to
ucrtbase.