sched.h doesn't exist...

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=115

Reported by David Hedbor, Idonex / Real Networks david@hedbor.org

On my Irix 6.2 system, sched_setscheduler is found but sched.h is missing. After changing it to include sys/sched.h, it compiles and links although a number of functions are not previously defined.

Suggested solution:

Use #if HAVE_SCHED_H (already checked for) and #if HAVE_SYS_SCHED_H if the first is missing.