Use the correct macro _WIN32 for Windows.

Use the correct macro _WIN32 for Windows.

The correct predefined macro for Windows is _WIN32, not WIN32 or
__WIN32__.  _WIN32 is defined for 32-bit and 64-bit version of Windows
by both MSVC and MinGW compilers.

http://sourceforge.net/p/predef/wiki/OperatingSystems
http://msdn.microsoft.com/en-us/library/b0084kay.aspx

This patch also fixes a MinGW automake build error.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>