configure script will now define HAVE_CLOCK_GETTIME_MONOTONIC symbol only

configure script will now define HAVE_CLOCK_GETTIME_MONOTONIC symbol only
when function clock_gettime() is available and the monotonic timer is
also available. Otherwise, in some cases, librt or libposix4 could be used
for linking even when finally not using the clock_gettime() function due
to lack of the monotonic clock.