This is another old option that doesn't make sense as a configurable.
So the option has been removed, and the check for AC_SEARCH_LIBS moved
into the main configure.ac file.

libphobos/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Use AC_SEARCH_LIBS for pthread_create.
	* m4/druntime/libraries.m4: Remove DRUNTIME_LIBRARIES_THREAD.