libstdc++: Fix condition for gthreads-timed effective-target

libstdc++: Fix condition for gthreads-timed effective-target

The refactoring in r11-5500 altered the condition for the gthreads-timed
test from #if to #ifdef. For some reason that macro is always defined,
rather than being defined to 1 or undefined like most of our autoconf
macros. That means the test always passes now, even for targets where
the macro is defined to 0 (specifically, Darwin). That causes some tests
to FAIL when they should have been UNSUPPORTED.

This restores the previous behaviour.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp (check_v3_target_gthreads_timed):
	Fix condition for _GTHREAD_USE_MUTEX_TIMEDLOCK test.