Simple changes to be homebrew friendly

[seclorum]

..

[seclorum]

Hi - no worries, I was going to do that but didn't think that copying the entire file just for a 3-char delta would be necessary, and also I'm sort of the opinion that the file should be more appropriately called "Makefile.Darwin" and it should, itself, figure out whether to use macports or homebrew, accordingly. I'll do that if you think its appropriate.

[pakozm]

Thanks @seclorum , I agree with your proposal, it would be so much better if you can do it :+1:

[seclorum]

Okay, added MONGO_LIBS_DIR as well as a safety-check to explain whats going on.

[pakozm]

Nice. Thanks!!