[RFC] Simplified Homebrew formula

Removed unnecessary cmake modifications, and manual building of
dependencies. Updates to Makefile remove the need.
@jszakmeister FYI, I was about to update the wiki, but `make CMAKE_EXTRA_FLAGS="-DCMAKE_PREFIX_PATH=#{prefix}" install` seems to use my cached cmake configuration, where as an explicit `cmake -DCMAKE_INSTALL_PREFIX:PATH=~/foo/` does not. 

So we should leave the wiki as it is to avoid confusion, but is this something that may also be a problem for homebrew users?