ProcessUNIX: Request the necessary SUSv2 features.

Some functionality used in ProcessUNIX requires SUSv2 features for
signal handling and select() functionality not available in ANSI C,
ISO C, or strictly POSIX.  Defining the _XOPEN_SOURCE feature test
macro ensures that the necessary functionality will be available.

See CMake bug 0015065 on Solaris:
http://www.cmake.org/Bug/view.php?id=15065

Change-Id: I5ea36feccafa32f2acd9903cf221335b4d7026ae