README: lose extra build vars from sandbox example
README: lose extra build vars from sandbox example

Rethink on this example. Packagers should probably be building MyPaint with
only the standard options, whatever those currently are. It's pointless
including extra options for flavour, and only distracts the reader from the
matter at hand in this para.

achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint