src: refactor vcbuild configure args creation

remove a bunch of variables and rely on %configure_flags% where
possible, also allow for an external %config_flags% variable to supply
additional arguments to configure to match the behaviour of the Makefile

PR-URL: https://github.com/nodejs/node/pull/3399


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>