* configure.ac: When using AC_SEARCH_LIBS, handle the case in which
that function returns "none required". Also, save and restore LIBS
around the check, and properly m4-quote the first argument to AC_DEFINE.
Finally, also set and AC_SUBST LIB_CLOCK_GETTIME.
This added code is very similar to that provided in gnulib's
clock-time.m4.
* tests/Makefile.am (LDADD): Append $(LIB_CLOCK_GETTIME).