Update SConstruct to support target selection

[CelticMinstrel]

Currently, running scons builds everything - the game, the editors, the tests, and (on Mac and Windows) a distribution package. That is, to say the least, a bit excessive. It would be better if you could select the target to build on the command-line, for example scons game builds only the game, scons pcedit builds only the character editor, scons tests builds and runs the tests, scons package builds a distribution package, etc. We could also in principle add scons install which actually installs the app to standard locations on your operating system (for example, on the Mac, copying it to /Applications or ~/Applications).

[clort81]

SConstruct also breaks with python 3.x. Needs 2.x. scons can be forced to use python2 by editing the first line of /usr/bin/scons.

[madwizard]

It's quite easy to move SConstruct to python3, question is, how will CI react to that?

[CelticMinstrel]

Turns out I created this issue twice – it's a duplicate of #52.