configure: don't clobber existing $PYTHONPATH

My `node` executable happens to be a wrapper created by 0install, implemented in python. It relies on some library code being available on `$PYTHONPATH`. Which means it can't run when `npm-gyp` overwrites `$PYTHONPATH` to include _only_ the bundled `gyp/pylib` dir.

This commit achieves the same effect as the existing code (giving the bundled `gyp` module priority), but without clobbering an existing `$PYTHONPATH` value.
Funnily enough, this would be a good use-case for my [path-array](https://www.npmjs.org/package/path-array) module.