Add build support for Mac OS X in Makefile

The 'cryptopp' library can't build with clang, the default compiler
on Mac OS X 10.8. To work around this I use the gcc compiler for the
'cryptopp' files and clang for the rest.

To install standard 'gcc' on Mac OS X using homebrew:

  $ brew tap homebrew/dupes
  $ brew install apple-gcc42