Ports/.port_include.sh, Toolchain/BuildIt.sh, Toolchain/UseIt.sh
have been left largely untouched due to use of Bash-exclusive
functions and variables such as $BASH_SOURCE, pushd and popd.
Much redundancy is removed from package scripts with this system.
It also supports simple dependency management, uninstalling (through
BSD ports style plist files), cleaning up after itself (with clean,
clean_dist, clean_all commands), etc.
I didn't look into why, but for some reason the SDL2 cmake build system
thinks it should build against PulseAudio which we definitely don't have.
So just tell it explicitly not to do that.
Fixes#265.
