build: unbreak configure with python 2.6

Commit 2b1c01c2 ("build: refactor pkg-config for shared libraries")
from May 2015 introduced python 2.7-specific code.

It mainly affects people building on old RHEL platforms where the system
python is 2.6.  Seemingly a dying breed because the issue went unnoticed
(or at least unreported) for a whole year.

Fixes: https://github.com/nodejs/node/issues/6711
PR-URL: https://github.com/nodejs/node/pull/6874


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>