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.
```
#if defined(__WXMSW__) || defined(__clang__)
#define MSG_NOSIGNAL        0
#endif
```
I don't know of a direct define for this.