0.12.7dev: improve `make status` output

 - display installed versions of Genshi, Babel, ...
 - display extensions to the PYTHONPATH and PATH; in r12168, I hinted
   at setting `path.$(python)` and `pythonpath.$(python)` in
   Makefile.cfg.sample, but actually didn't move the corresponding
   code from my Makefile.cfg to the sample; now it's done directly in
   Makefile, see `PATH-extension` and `PYTHONPATH-extension`
 - added some concrete example in Makefile.cfg.sample


git-svn-id: http://trac.edgewall.org/intertrac/log:/branches/0.12-stable@13248 af82e41b-90c4-0310-8c96-b1721e28e2e2