ENH: tuning wmake behaviour (#1647)

- preferentially handle Allwmake.override, which allows packaging
  tools to define alternative make scripts, or selectively disable
  components.

- remove legacy handling of 'Optional' directory.
  Conditionals have since migrated into scripts themselves and/or
  use the wmake/scripts/have_* framework.

BUG: missed passing -debug for Allwmake scripts