ENH: reduce wmake bootstrap dependencies

- remove make target for wmdep (flex-based scanner), which eliminates
  a bootstrap dependency on flex.

  As of OpenFOAM-v1806, wmdep has been superseded by wmdepend
  (ragel-based scanner).

- replace dirToString binary with shell/awk equivalent for simpler
  maintenance. The utility is very rarely used (auto scanning to
  create Make/files) so there is no performance difference.