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


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@15460 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b