Use Windows threads library with mingw

Instead of Posix threads. This seems to fix time
losses of the gcc compiled version for Windows.
The patch replaces the MSVC specific _MSC_VER flag
with _WIN32 and _WIN64 that are defined both by
MSVC and mingw-gcc.

Workaround found by Jim Ablett.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>