Fixes for building dogecoind on OSX

When building dogecoind on OSX I get this error with the [dogecoin](github.com/dogecoin/dogecoin) codebase:

    ld: warning: directory not found for option '-L/opt/local/lib'
    ld: warning: directory not found for option '-L/opt/local/lib/db48'
    ld: library not found for -ldb_cxx-4.8
    collect2: ld returned 1 exit status
    make: *** [dogecoind] Error 1

The version of berkeley db that brew installs is 5.3 so I've made changes to support it. I'd be happy to make modifications if this isn't quite the ideal solution. I'm not extremely familiar with make so there might be some issues with this approach.