Add unified Python build script for PhantomJS.

This script merges the functionality of all these files:

build.sh
src/qt/preconfig.sh
build.cmd
src/qt/preconfig.cmd

Note that some recent changes to the above files are not yet
included in this script. Due to that, these files are not yet removed.

If you fear this pulls in Python as another build dependency, then
fear not: Qt WebKit already requires Python internally. It also
includes Ruby and Perl btw, but I opted for Python.

https://github.com/ariya/phantomjs/issues/12580