Fix glibc-sched.h check for _GNU_SOURCE
Fix glibc-sched.h check for _GNU_SOURCE

We can't check that sched.h was included earlier with _GNU_SOURCE
by looking at CPU_SETSIZE because it doesn't exist on redhat 8.
Look for sched_priority as well, that's probably the only other
interesting #define in sched.h.

git-svn-id: 14be032f8f42541b1a281b51ae8ea69814daf20e@4061 4b44e086-7f34-40ce-a3bd-00e031736276

BlueBrain/hwloc,BlueBrain/hwloc,BlueBrain/hwloc,BlueBrain/hwloc