build: Remove compatibility check for clock_gettime().

Only Mac OS (which is no longer supported) did not have it. Moreover,
on Linux we used to check whether one needs to link with -lrt to
obtain clock_gettime(), which is necessary for glibc versions 2.16
and earlier (released in 2012). All distributions should have moved
on to newer glibc versions by now, so let's get rid of the check.