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.
On OS X, the compiler errors because of multiple definitions. I can put the guard around the parameter definition if you'd prefer (as that's the only difference) and delete the duplicate body. Maybe there's a way to define the parameters correctly for all platforms, but again, I have no interest in figuring that out, I just made a few small changes to get it to compile on OS X.