Support make install to user-defined prefix
This will mainly affect the meta script and PLAYIT_LIB2 detection and will help with Debian packaging (though currently I have a workaround).
I recommend installing to $PREFIX/share/games/play.it/ and the bin to $PREFIX/games/play.it. That works well with /usr or /usr/local, though I admit it doesn't work well for ~/.local. I also don't think it's necessary to install the src and patches dirs.