CC: Proper "package manager" for C

[obiwac]

Currently you can already kinda add "packages" with Deps.git, but it would be cool to have some kind of central repo with ports for a bunch of different libraries, which can simply be added like this:

cc.add_dep("shapelib") // simply add the dependency, no extra fuss
cc.add_dep("shapelib", "0.1.0") // specific version of shapelib

There could then be like a GitHub org (like https://github.com/bob-deps) which houses all of these ports (and releases).

I don't know if it should be global or only on cc (like it is with rustc.add_dep currently), there're a few things still left to figure out.