…` flags,

ocamlbuild options) and bugfixes (to pack production, for 4.08+dev support).

- #45, #190: add ppopt(arg) and ppxopt(package,arg) when -use-ocamlfind
  (Gabriel Scherer, review by whitequark,
   request by Gabriel Scherer, Gabriel Radanne and Pavel Argentov)

- #268, #269: add flag support for some ocamlmklib options:
  custom, debug, failsafe, linkall, ccopt(..), cclib(..), rpath(..), ldopt(..)
  (Gabriel Scherer, report by Hannes Mehnert, review by whitequark)

* #272: add the "link" tag to "pack" actions
  Instead of a separate category, "pack" is now another form of linking
  like "program", "library", "toplevel" and "output_obj". This fixes
  the issue that package(...) tags where not passed at pack-production
  time, spotted by Jérémie Dimino. More generally, this extends
  the meaning of all "link" flags to "pack", which seems to be the correct
  behavior for all the rules we inspected.
  (Gabriel Scherer, original issue diagnosis by Jérémie Dimino)

- #278: typo fixes in the manual
  (Xinzhe Yang)

- #282: fix compilation with trunk OCaml (4.08+dev)
  (Xavier Clerc and Nandor Licker)