This was not working properly and needed some external build script as well as
the stamp/bogus header trick like for other similar in-source generated code.
In the same time, I get rid of old meson code which was meant for when using
meson < 0.57.0 (since our requirement is now meson >= 0.59.0).
The enums have to be menually registered in pdb/enums-external.pl.
Currently supports enums from GEGL only.
Add enum GeglDistanceMetric as first external enum.
It never belonged inside "tools". Also rename its "pdb" subdirectory
to "groups". This had to happen before 2.10 so cherry-picking between
branches doesn't become a nightmare in the future.
Jehan