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.
Eg gettext gets added into LDFLAGS, INCLUDE and that. I hope I got everything
that is typical. Prolly not. But we'll find out.
Made readline keg_only because the BSD version is provided by OS X, and I
don't want bug reports that are tricky to solve due to unexpected differences
between the two.
