Minimal changes to get building on OS X with clang and Homebrew

Install dependencies with Homebrew and then from within src directory:

    make USE_UPNP=1 -j $(sysctl -n hw.ncpu) -f Makefile.osx all

The only concern is how to install specific versions of the dependencies (to avoid accidentally resulting a hard fork).

Unfortunately, I couldn't find "db-4.7.25.NC" via Homebrew. The farthest back they have is 4.8.30. Is that OK?

Also, I haven't tested yet if boost 1.5 builds on OS X or not. 1.55 does, but I'm not sure about 1.5.
> Just remove the correct duplicate and it should work with any compiler on any OS.

Removing one will break compilation on one of the platforms (regardless of which you remove), if I remember correctly.

Feel free to send your own PR on top of this one if you'd like.

> Then why bother at all if you only want a quick and dirty hack?!

...I wanted "minimal changes" to get it to work on OS X. I got them.