Turn off threading support if glibc++ is single threaded.

Enable threading support on HPUX for gcc >= 3.3.


[SVN r26967]

include/boost/config/stdlib/libstdcpp3.hpp

@@ -39,6 +39,9 @@
 #  else
 #     define BOOST_DISABLE_THREADS
 #  endif
+#elif defined(__GLIBCPP__) && !defined(_GLIBCPP_HAVE_GTHR_DEFAULT)
+   // disable thread support if the std lib was built single threaded:
+#  define BOOST_DISABLE_THREADS
 #endif