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.
Establish build order, don't let scons guess

Some combinations of the build/rebuild cycle occasionally cause build
ordering screwups still. Try establishing a definite order.
OS X: remove workaround for python<2.7.4

Remove some OS-specific workaround clutter which can be worked around by
installing a newer Python now that http://bugs.python.org/issue16848 is
fixed and in widespread deployment (H/T @ryandesign in #52).

The current Apple-supplied Framework Python is 2.7.8, same as Macports.
SConstruct: different openmp defaults for OS X

We already do this for the prefix, so it's fine to do it for OpenMP support
too. The defaults can of course be overridden when clang adds support for
OpenMP, or when compiling with compilers that support -fopenmp.

Fixes #55.
SConstruct: turn off '-z origin' fully for OSX

I was hoping we'd be OK with it behind -Wl, but linking reportedly fails
even when wrapping it up in -Wl,-z,origin on Darwin. Hopefully lack of it
won't break installs.
Update docs with OS X build notes

Preliminary build notes tested on Mavericks. Thanks to Stuart for the testing and patches!
README: minor clarifcation