Ctypes 0.2.
Add version constraint to ctypes package.
Add depexts for Ubuntu.
Merge branch 'ctypes-0.2' of git://github.com/yallop/opam-repository into yallop-ctypes-0.2
Also restrict older ctypes to OCaml>4.00.0, as they depend on hash.h
Merge branch 'master' into yallop-ctypes-0.2
Merge branch 'master' into yallop-ctypes-0.2
Merge pull request #1280 from mirage/yallop-ctypes-0.2

ctypes 0.2 with older versions restricted