This is equivalent to the previous situation, but more easily
maintainable.
* acinclude.m4 (gl_CLOCK_TIME): Remove.
* configure.ac: Remove explicit use of `gl_CLOCK_TIME'.
* m4/gnulib-cache.m4 (gl_MODULES): Add `clock-time'.
* gnulib-local/m4/clock_time.m4.diff: New file.
* Makefile.am (EXTRA_DIST): Add it.