Under OSX 10.11.6 I'm getting a few errors of the kind:
csdr.c:2080:31: error: use of undeclared identifier 'CLOCK_MONOTONIC'

I'm using macports to provide the build system, and I had to modify line 47 in the Makefile to add the /opt/local/include and /opt/local/lib directories, else libfftw wasn't being found.

Not sure if this is a compile issue (gcc isn't really gcc... it's LLVM), or something else.

It looks like clock_gettime and CLOCK_MONOTONIC were only added in OS/X 10.12 Sierra. I pushed this solution, which works on Sierra. Let me know if it works for you @darksidelemm.

It builds fine now! Not able to do any serious testing at the moment, but that's a good start!
(Still have to manually specify the include and lib directories mind you, but i'm not sure what the right solution to that is)

Why is this change not being merged?

Building works under macOS Mojave, but installing fails:

Michaels-MacBook-Pro:csdr user$ sudo make install
install -m 0755 libcsdr.so.0.15 /usr/lib
install: /usr/lib/libcsdr.so.0.15: Operation not permitted
make: *** [install] Error 71

Changing the PREFIX in Makefile to /usr/local worked for me.