This is bug to make sure the issue in bug #145241 gets resolved. The postgres and mysql support (plus any other optional auth) should be built as loadable modules as described in the dovecot INSTALL doc file. Then these modules should be provided as sub-packages. The rational is dovecot should be lightweight and not have heavy external dependencies. If a site wants the optional mechanisms then they can install an appropriate sub-package that provides a loadable module with the support and only then will there be a dependency on external packages.
Since all the discussion happened in bug #145241, i am closing this one as a dupe and retitling the former to better describe the problem at hand. *** This bug has been marked as a duplicate of 145241 ***