Draft: Add makefile

This is a very WIP branch where I started moving things to support make. This will hopefully simplify packaging librem5-agps for other distros that want to use it. As you can see, there are still a few TODOs that need to be addressed before this is ready for review, so don't spent too much time on reviewing it yet. But definitely let me know if any of the big changes here are heading in the wrong direction.