Build: Use separate environments for _mypaintlib and brushlib
Build: Use separate environments for _mypaintlib and brushlib

brushlib does not depend on all the same things that _mypaintlib
does, so it should not be built in the same construction environment.

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