Now we no longer rely on anything from Boost. Later we'll move the path
detection to the very beginning of the plugin bridge initialization so
it can be set from the coming chainloader libraries.
We sadly cannot call `shared_library()` and `executable()` in these
subdirectories while still maintaining the same `build/` directory
structure, but this is still much cleaner. All of the other build
artifacts are now also gone from the root of `build/` so it's cleaner
overall.
Robbert van der Helm