WIP/RFC: add cmake build system

Addresses #1832.

 - [ ] build Julia on one platform (*nix) with one configuration (whatever you get when throwing the fewest flags)
   - [x] `libsupport`
   - [x] `libmojibake`
   - [ ] `libjulia`
   - [ ] the `julia` exe
 - [ ] extend to:
    - [ ] all platforms
    - [ ] multiple configurations
    - [ ] dependencies
    - [ ] the `dist` target