Add _spawnxxx Macros for MSVC Compatability

MSVC has deprecated the POSIX-ish calls spawnxxx in favor of ISO standard _spawnxxx calls.  While I don't think we should remove the original calls, I think some macros to provide the equivalent calls for OpenWatcom might be in order.

I'm not sure if this commit is sufficient for everyone.  However, it does fix the issue in a simple manner.