Reduce number of prints in --help mode
More central and portable removal of old targets

Removal of build targets from ancient MyPaint trees is fairly dubious, but
it's a project-level hack. Centralize it the root SConstruct, and make it
more portable.
Document scons build dependency, nominate version

Dependency on scons wasn't documented. Fixed.

Nominate version 2.1.0 as the minimum supported level. That's in Ubuntu
12.04LTS (like, the *old* LTS); also, scons 2.1.0 is required for mingw
builds, according to https://gna.org/bugs/?21228

The current scons version is 2.3.2 at the time of writing, and that's what
I mostly develop against.
Swig: use native support provided by SCons

SCons version 2.1.0+ appear to support swig just fine, and provided you're
careful about declaring inter-module dependencies the resultant project
build order is correct too.

* Use SCons's native swig support
* Move the _mypaintlib.so → brushlib dependency into lib/ where it belongs

Builds work correctly after a --clean, over many calls with --random, and
rebuild correctly following changes in source deps correctly. Versions of
scons tested: 2.1.0, 2.3.1.

MyPaint now doesn't try to call scons until after --help has been
processed, so this commit closes #54.