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.
Mac OS X projects that use SDL through Objective-C may
require the "SDLmain" support files.
Since these are not strictly header files, they are not
installed into include by default, so we put them in our
libexec folder.
