[CMake] -lrt finder

[Davis Ford]

I'm building for Raspberry Pi - which is embedded Linux on Arm, distro
is Debian-based.

I'm getting this linker error:

libzmq.a(libzmq_la-clock.o): In function `zmq::clock_t::now_us()':
/home/pi/git/libzmq/src/clock.cpp:76: undefined reference to `clock_gettime'

I believe I need to add "-lrt", I tried to look through the list of
Finder modules, but I can't seem to identify the right one -- if one
exists.  What is the correct way to solve this?