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.

This commit was SVN r4061.

ggouaillardet/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc,ggouaillardet/hwloc,ggouaillardet/hwloc,ggouaillardet/hwloc,shekkbuilder/hwloc