postfix: add build options

[ailiop-git]

Enable build to be more modular by making most of the dependencies
optional. Specifically, mysql, pgsql, sqlite, ldap, sasl and pcre are
not strictly required and are all optional dependencies.

General

• This is a new package and it conforms to the quality requirements

Have the results of the proposed changes been tested?

• I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
• I generally don't use the affected packages but briefly tested this PR

[ericonr]

You want to revbump it so metadata is updated,

[github-actions]

Pull Requests become stale 90 days after last activity and are closed 14 days after that. If this pull request is still relevant bump it or assign it.