configure: also check for clockgettime()

clock_gettime() is also in -lrt so we also need to
check for it.

Use AC_SEARCH_LIBS() instead of our canned combo of
AC_CHECK_FUNC() + AC_CHECK_LIB(). AC_SEARCH_LIBS()
will automatically add the necessary -l flags to the
LIBS variable, so we don't need out AM_CONDITIONAL()
construct either, now.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>