BUILD: Makefile: add USE_RT to pass -lrt for clock_gettime() and friends

Some code will require clock_gettime() which needs -lrt on most Linux
distros (those with glibc < 2.17). For this reason, this patch introduces
USE_RT to enable -lrt, which is implicitly set for all Linux flavors,
since it's harmless to link with it on more recent ones. Those who know
they can safely get rid of -lrt can remove it using "USE_RT=".
1 file changed