Defining _XOPEN_SOURCE at configure-time defines HAVE_UCONTEXT_H.  This causes a faulty build somewhere.

Mono builds functionally on OS X without _XOPEN_SOURCE being defined, so we'll remove it for now. We have not yet verified that this doesn't generate other problems, but this should help us test.

Also note that _XOPEN_SOURCE has always been defined for bockbuild Mac packages.