build: Make configure file parseable on python3

Display python3-compatible error message for some systems use python3 as
default.

PR-URL: https://github.com/nodejs/node/pull/9657


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>