Build system update

[BlakeFreer]

Todo

• SIL platforms for demo projects
• all other projects

who is responsible for linking mcal to main? rn the platform cmakelists does it but main is fully capable now that we don't have a bindings target. we could omit the mcal variable in mcal_conf and by default, include all of the mcal-* libraries, and then selectively link one in the platform cmakelists. this only works if cmake understands not to compile or parse the unselected ones