build outside source tree

[bdod...@gmail.com]

Hello, I've worked with phantomjs a number of years ago and made a number of improvements.  I am about to do the same type of work again, and it involves building a number of different ports of the code to various platforms.  I found I really need the ability to build outside of the source tree with various cross-compilers, etc..  I made a mod to the build.py to add a "--build-prefix=<path>" argument and have it working.  I'd like to start contributing my mods, starting with this, back to master here (after making issues, reviews, etc.) and will need permission to push commits to feature branches.  Thanks!  Brian