Add option to specify alternative location for chipmunk source

[FinnG]

I'm using this in a project I'm working on and it seems like a neat thing to have in general.

Default behaviour is unchanged, but if the user doesn't want to put the chipmunk source in a directory called "chipmunk" they can instead build with the following command line:

scons chipmunk_dir="<path_to_built_chimpunk_source>"

[jhasse]

Looks great! Thanks :)