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.
Adam Vandenberg
Max Howell