merge lp:~gary/zc.buildout/python-support (also available as svn+ssh://svn.zope.org/repos/main/zc.buildout/branches/gary-launchpad).  This adds support for system Pythons, as well as other changes, such as improved Distribute support and some cleanups.

git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@111588 62d5b8a3-27da-0310-9561-8e5933582275