Build: require libqb 0.17.0 or later

This allows us to drop some compatibility checks.

Also take the opportunity to straighten out our ACL support checks. Previously,
we required libqb 0.11.0 or later, and tested for a function defined in 0.14.0
to help determine ACL support. However, the code used the function
unconditionally, meaning we actually required 0.14.0 in practice. Now, we can
just assume it's there, which simplifies the configure.ac checks considerably.