Fix to make gem compile on Arch Linux (second attempt)

I experienced the same problem mentioned in issue cowboyd/libv8#61. It seems that Arch Linux (and perhaps other systems?) requires some changes to the V8 source to make it compile using the 'python2' executable instead of the assumed 'python' executable (which on Arch points to Python 3). The proposed patch is copied from the Arch Build System (ABS), and is executed whenever a 'python2' executable exists on the system.

This is a revised version of pull request #64 (the previous version failed with 1.8.7).