[4.8] Enable _GLIBCXX_HAS_GTHREADS in GCC 4.8

Fix a test in libstdc++/confiugre to include $glibcxx_thread_h instead
of gthr.h otherwise it will fail to detect gthread lib because the
new gthr.h only include gthr-default.h which doesn't exist in source
directory

See issue http://code.google.com/p/android/issues/detail?id=41770
946c2a6b57ac7d3b2b656ac628bcb14a216ec635

Change-Id: I13bd69f45ddee8b38055f0264c8aa484df0761d2

Author: Andrew Hsieh <andrewhsieh@google.com>
Date:   Tue Apr 2 19:41:43 2013 +0800