Provide glthread/lock.h implementation in terms of Guile threads.

Provide glthread/lock.h implementation in terms of Guile threads.

Fixes <http://bugs.gnu.org/14404>.
Thanks to Paul Eggert <eggert@cs.ucla.edu> for the suggestion.

* m4/gnulib-cache.m4: Avoid 'lock' module.
* m4/lock.m4, m4/threadlib.m4, lib/glthread/threadlib.c,
  lib/glthread/lock.c: Remove.
* lib/glthread/lock.h: Rewrite in terms of libguile/threads.h.
* m4/gnulib-common.m4, m4/gnulib-comp.m4, lib/Makefile.am,
  lib/unistd.in.h, maint.mk : Update, from Gnulib v0.1-92-g546ff82.

* configure.ac (GNULIB_LOCK): Define 'GNULIB_LOCK'.  Define
  'USE_POSIX_THREADS' when building with pthread support.
* Makefile.am (noinst_HEADERS): New variable.
  (BUILT_SOURCES): Add libguile/scmconfig.h.
  (libguile/scmconfig.h): New target.