Buildscr: make long parts of the build conditionalisable.

Buildscr: make long parts of the build conditionalisable.

If I want to rebuild just the Javascript puzzles (for example) in
circumstances where I don't expect to need a great many
edit-compile-link cycles, it's easier to get bob to do it for me than
to remember how to set up the development tools on my path. But it
takes ages to run the whole build script if I also have to wait for
the Windows, Mac and Java puzzles to be built, not to mention the
initial Unix build that runs for no purpose other than generating the
icon images.

So now I can run the build with various time-consuming parts
conditioned out, for development purposes. Of course, the default is
still to build absolutely everything.