Check _GNU_SOURCE before using get_nprocs.

The latter function is not POSIX, but a GNU extension and therefore
not available universally.


[SVN r54408]

libs/thread

@@ -1 +1 @@
-Subproject commit 627cb7f77450fdb9ae1376ba6da6d0344b97aaac
+Subproject commit 235ed4afe00dc96a00841ab4a3156c284715777d