threadlib: Fix LIBMULTITHREAD on platforms where --as-needed is enabled.

threadlib: Fix LIBMULTITHREAD on platforms where --as-needed is enabled.

Reported by Erik Auerswald <auerswal@unix-ag.uni-kl.de>
in <https://lists.gnu.org/archive/html/coreutils/2018-06/msg00063.html>.

* m4/threadlib.m4 (gl_THREADLIB_BODY): Check whether the linker supports
--as-needed/--no-as-needed and --push-state/--pop-state. When defining
USE_POSIX_THREADS_WEAK or USE_SOLARIS_THREADS_WEAK or
USE_PTH_THREADS_WEAK, define LIBMULTITHREAD in such a way that -lpthread
/ -lthread / -lpth does not get optimized away by a preceding
--as-needed option.