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.
Xerces-C++ is a validating XML parser written in a portable subset of C++.
Xerces-C++ makes it easy to give your application the ability to read and
write XML data.
Adam Vandenberg
David Höppner
Max Howell