PR libstdc++/52189
	* acinclude.m4 (GLIBCXX_CHECK_GTHREADS): Handle
	--enable-libstdcxx-threads.
	Disable on Solaris 8/9 with symbol versioning.
	* configure.ac (GLIBCXX_CHECK_GTHREADS): Move after
	GLIBCXX_ENABLE_SYMVERS.
	* configure: Regenerate.
	* doc/xml/manual/configure.xml (--enable-libstdcxx-threads): Explain.

From-SVN: r184335