build: use %PYTHON% instead of python

Like libuv does.

Originally: https://github.com/node-forward/node/pull/21

PR-URL: https://github.com/iojs/io.js/pull/1444


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

Conflicts:
	vcbuild.bat