When running ./package.sh to rebuild an already installed port, we would not
want to spend time re-downlodaing the same tarball again. Ideally, this should
use some sort of hash checking to ensure the file is not truncated or something,
but this is good enough for now.
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.
This way, we don't (in the ports themselves) depend on perl as a public
interface, which means if we ever have to, we can port to something else easier.
