Log Message:
Update to 20170501 (v15.x, "Phosphorus"). This reflects six major
upstream releases, so visit the HOMEPAGE for further info.

pkgsrc changes:
   - old patches were rolled in upstream
   - use the ocaml framework
   - depends on more ocaml libraries
   - depends on lang/coq by default; turn off the coq option to avoid this

XXX: You must build ocamlgraph with ocaml-lablgtk support (which is
XXX: not the default) or the build fails on missing module "Dgraph".