build postgresql on Mac OS X mountain lion with ossp-uuid

Hi,

On the new Darwin 10.8 (aka mountain lion), I had to add

#ifdef __APPLE__
#if (__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 1080)
#define _XOPEN_SOURCE
#endif
#endif

to the very beginning of contrib/uuid-ossp/uuid-ossp.c to get the build
working with --with-ossp-uuid.

I suggested something similar to the macports guys, but I guess it would
be wise to get this upstream to the postgresql source?

Cheers,
Palle