Improve Makefile for MacOS

Some macs appear to need "-framework CoreFoundation" added to the linking
step, others (which appear to have the exact same OS and tools installed),
don't. But as it doesn't appeart to hurt, I unconditionally add this.

Switched to using pkgconfig to find libdivecomputer on the Mac.

Tried to clean up the Makefile a bit

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Acked-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>