Conditionally delegate targets to subdirectories

If a target is defined by the user, it should not be delegated to
subdirectories.
Remove METAPOST leftovers mptextmp.mp and mptextmp.mpx

Closes #19
Fix preparation of METAPOST recipes

Closes #12
Prepare patchset/v2.0.1

- issue/12
- issue/15
- issue/19
Merge branch 'patchset/v2.0.1' into maintenance/v2.0
BSD Make Pallàs Scripts v2.0.1