The _POSIX_PRIORITY_SCHEDULING macro is specified by POSIX to indicate that the
implementation supports the Process Scheduling [PS] option group, documented
here:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_08_04

On Linux, glibc defines _POSIX_PRIORITY_SCHEDULING, but does not implement the
Process Scheduling functionality of POSIX; Linux does not even support the
required functionality.

What's worse, glibc DOES provide functions with the same names as the functions
in the Process Scheduling group (sched_*), but the glibc semantics for these
functions are radically different from those specified in Process Scheduling.
In particular, the glibc functions do not take a process id as their argument;
they take a kernelspace tid, which is not even available to applications
because glibc does not provide gettid(). Even if passed the pid (which is also
the tid of the main thread), these functions only change the scheduling
properties for a single thread, not for the process. Linux does not even
support process-level scheduling options, only thread-level ones.

Since it doesn't seem realistic to remove the erroneous functionality the
sched_* functions are providing now, I think a reasonable solution would be to
redefine _POSIX_PRIORITY_SCHEDULING to -1 or 0 and have sysconf return -1 for
it. If glibc does not advertise support the the Process Scheduling option, then
it's under no obligation to provide the standard sched_* functions, nor to
avoid providing conflicting functions with the same names.