So many tools expect all SDL modules to be installed in the same place. So we have to change the prefix path to HOMEBREW_PREFIX. This is probably the correct thing to do in general in fact.
brewkit.rb changes ENV destructively, so lets not do that everytime a formula
is required. Now it's possible for other tools to require a formula
description without worrying about side-effects.
We depend on newer libpng than OS X provides because the libpng version has
bugs that exhibit themselves for some uses of the SDL.
See issue Homebrew/homebrew#89 for more information.
