On MacOS and (I think) most BSDs the default standard library is libc++, and so attempting to link with libstdc++fs does not work.

On Linux, where (AFAIK) most distros use Clang/libstdc++, this is going to mean link-time failures when using std::filesystem. In this case the user can add Link-Flags: -lstdc++fs to their toolchain file themselves as a fix.

This is intended to fix #11.

Unfortunately I haven't been able to successfully build DDS yet to actually try it out, but looking at the surrounding code it seems like the right solution...

- Build execution is now entirely oriented around source distributions.
- Make a lot of build plan objects and methods more agnostic to the
  build environment. This allows more flexibility and delays generation
  of some build metadata, such that we are able to more easily generate
  usage requirements data between libraries in the project.