Merge pull request #458 from avsm/master

jenkins update
Merge pull request #460 from avsm/master

jenkins: srs
Rebuild the metadata cache when adding/removing/changing the priority of repositories.
Do not fail badly if opam-lib is not in list of available packages.
Fix both #463 and #446

The error messages are now much clearer:
* in case of a dependency is not available due to compiler/OS constraints
* in case the request deals with non-existing package/version
Merge pull request #462 from avsm/master

jenkins: oqamldebug
Merge branch 'master' of github.com:OCamlPro/opam
Fix #455

We now have a more precise OPAM and OS versions in error messages
Fix infinite loop introduced in the last commit
Rename OpamGlobals.default_switch and OpamCompiler.default to system (as this is the name of the string anyway).
Add a way to specify system compiler constraints.

The syntax is:

ocaml-version: [ !system ]
ocaml-version: [ "3.12.1" || system ]

Fix #461
Update CHANGES
Fix build (I've updated the .ml instead of the .ml.in ...)