Introduce a mechanism in Buildscr for optionally building some of the

Introduce a mechanism in Buildscr for optionally building some of the
(more finished) puzzles in 'unfinished', as Java applets only. (The
rationale being: puzzles in 'unfinished' can be played locally by
people who go to the extra effort of downloading and building the
source, but to play them in Java is particularly inconvenient unless I
build the Java version myself. I just won't link it from the front
page.)

[originally from svn r9073]