Merge pull request #199 from euanh/build-improvements

Update ocaml-xen-lowlevel-libs and xenopsd to point to versions with Debian linker paths
ocaml-syslog: Switch to OPAM's source archive

The GODI URL from which we have been downloading this package
seems to have gone away.

Signed-off-by: Euan Harris <euan.harris@citrix.com>