[pkg-otr-team] Bug#767652: libotr5: Generate versionned dependencies for reverse-dependencies

[intrigeri]

>   * Generate and maintain a symbols file.

Note that this is enough if, and only if, we don't reintroduce
upstream's runtime version check (which we're going to remove for
Jessie). If we reintroduce this check, reverse-deps compiled against
a new version of the library won't be able to run against an older
version of the library, unless they use symbols introduced by the new
version (and thus explicitly depend on it).