Log Message:
x11/ocaml-lablgtk3: Add -verbose when using pkg-config

This doesn't fix the build, but it provides a clue: dune code is
objecting to the pkg-config that is found, which is a shell script
wrapper:

  File "src/dune", line 9, characters 0-128:
   9 | (rule
  10 |  (targets
  11 |    cflag-gtk+-3.0.sexp
  12 |    clink-gtk+-3.0.sexp)
  13 |  (action (run dune_config -verbose -pkg gtk+-3.0 -version 3.18)))
  (cd _build/default/src && ../.bin/dune_config -verbose -pkg gtk+-3.0 \ 
-version 3.18)
  which: /tmp/work/x11/ocaml-lablgtk3/work/.tools/bin/pkg-config
  -> not found
  configure error: pkg_config not installed

Take maintainership (offlist discussion with jaapb@).