brushlib: Allow to build gobject introspection bindings
brushlib: Allow to build gobject introspection bindings

This will allow the library to be used from Python, Vala,
Javascript and similar.
Enable by passing enable_introspection=true to scons.

b3sigma/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint