This will be used in particular for adding GimpTRCType to libgimp.
Otherwise it generates some broken gimp_TRCtype_get_type() function
name.
For this use case, we need to look-ahead a bit in the regular
expression.
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.