Fixes for building dogecoind on OSX 10.8.4 with homebrew

Cleaned up everything that was in [this previous pull request](https://github.com/dogecoin/dogecoin/pull/124). @somegeekintn and @shrx were helping on the previous request.

I am running OSX 10.8.4. There were some issues building on OSX 10.9. It is suspected that the issues are caused by different `g++` versions. Specifically:

on 10.8.4:

    clang --version
    Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
    Target: x86_64-apple-darwin12.4.1
    Thread model: posix

on 10.9.1:

    clang --version
    Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
    Target: x86_64-apple-darwin13.0.0
    Thread model: posix

Original pull request comment is below:

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.