Add build compatibility qalculate-backend with libqalculate < 2.0.0

Summary:
According to https://phabricator.kde.org/D7590 @filipesaraiva remove cln linking on commit 6ad659fc.
It's cause a lot of ld errors (undefined references), if you have libqalculate with version less 2.0.0.
I have changed this to link cln library depending on the version of libqalculate.

Reviewers: #cantor, asemke

Reviewed By: #cantor, asemke

Subscribers: #cantor, filipesaraiva, #kde_edu

Tags: #kde_edu, #cantor

Differential Revision: https://phabricator.kde.org/D11574