Fix GCC's use of _POSIX_PRIORITY_SCHEDULING and
_POSIX_THREAD_PRIORITY_SCHEDULING. POSIX 2001 states that they can be
defined to -1 to indicate that the feature is not supported (which is
how we def

Fix GCC's use of _POSIX_PRIORITY_SCHEDULING and
_POSIX_THREAD_PRIORITY_SCHEDULING. POSIX 2001 states that they can be
defined to -1 to indicate that the feature is not supported (which is
how we define them), so it's not okay to just check whether or not
they're defined.

Arguably, SUSv2 allows GCC's usage, but we don't special case these
feature flags for SUSv2, and these files need to be compilable in BSD
and POSIX compat modes anyway.

From Brad.

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import of gcc-4.2.1, the last gcc release under GPLv2