Restore build on Mac OSX >= 10.7 since the inclusion of ucontext.h

Hi,

Commit 5786be99c56091d63a4a5f377093a3db65144bce added the inclusion of ucontext.h, that requires _XOPEN_SOURCE to be defined to work properly on Mac OSX >= 10.7, where the build is therefore broken. This patch fixes this. Thanks to consider merging.

Cheers,
  Simon