Makefile: ease -lrt on the Linux systems that require it

AFAIK not all Linux systems require to link with -lrt (which is
used by libutp to access a monotonic clock).

For example, Slackware does not while Ubuntu does.

(The Slackware manpage for clock_getres(2) clarifies that one
needs to -lrt only for glibc before version 2.17.)

Also, MacOSX does not even have librt.

Therefore, add an knob that one can use to compile with librt.

A ./configure script would have been probably better, but I
don't have time to prepare it right now.
Merge branch 'fix_librt'