Improve SRCDIR/INCLUDEDIR handling; allow dot

- Explicitly allow `find` to match `.`, which otherwise would be
  excluded as a dotfile.

- Add special handling to drop the leading `./` that results from the
  above, and generate the appropriate local paths without needing it.

- Error when trying to perform certain transformations on file·names
  and failing, to better aid diagnosis.

- When a file has no destination (because `destinations` has not been
  generated yet), use the fake destination `NOTDEF`. (Make will restart
  before this destination is used.)