Uniformly define _XOPEN_SOURCE to 600.
The feature_tests.h header from Sun systems (Solaris/OpenSolaris) will abort
the build if _XOPEN_SOURCE is defined to 500, and C99 is requested (as well
as POSIX.1-2001), and will only accept it to be defined to 600.
inspired by a patch from Diego Pettenò, flameeyes gmail com
Originally committed as revision 15460 to svn://svn.ffmpeg.org/ffmpeg/trunk