cppo: Switch to GitHub sources

Signed-off-by: Euan Harris <euan.harris@citrix.com>
ocaml-easy-format: Switch to GitHub sources

Signed-off-by: Euan Harris <euan.harris@citrix.com>
ocaml-biniou: Switch to GitHub sources

Signed-off-by: Euan Harris <euan.harris@citrix.com>
ocaml-yojson: Switch to GitHub sources

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #561 from euanh/build-improvements

Build improvements