Fixes gyp formula to work with system Python

This avoids the following error noted at https://github.com/tessel/runtime#os-x:

> ==> Checking out http://gyp.googlecode.com/svn/trunk/
> ==> python setup.py install
> 
>   http://peak.telecommunity.com/EasyInstall.html
> 
> Please make the appropriate changes for your system and try again.

This is due to a simple permissions error — it's trying to install to the default (i.e. system-wide in this case) Python but can't write there. Fixing is relatively simple, just needs to be installed within Homebrew's prefix plus some other "older setup.py" workarounds as documented in <https://github.com/Homebrew/homebrew/wiki/Homebrew-and-Python#if-the-software-provides-a-setuppy>