* config.gcc: Don't accept --enable-threads=pthreads.  Clean
	up related case statements.
	* configure.in: Don't accept --enable-threads=pthreads,
	decosf1, mach, or os2 (none of which work anyway).  Alphabetize
	supported thread files in case clause.
	* configure: Regenerate.

From-SVN: r72244