This whole thing happens because the libreoffice 5.3 packages use versioned "Provides:", but apt in Ubuntu 14.04 is too old and does not support these yet.
According to the changelog, "versioned provides" where added in apt 1.0.7, but Ubuntu 14.04 ships with apt 1.0.1.
To solve this issue, the libreoffice packages for Trusty must either stop using versioned provides, or apt in Trusty must be updated.
This whole thing happens because the libreoffice 5.3 packages use versioned "Provides:", but apt in Ubuntu 14.04 is too old and does not support these yet.
According to the changelog, "versioned provides" where added in apt 1.0.7, but Ubuntu 14.04 ships with apt 1.0.1.
To solve this issue, the libreoffice packages for Trusty must either stop using versioned provides, or apt in Trusty must be updated.