Add build compatibility qalculate-backend with libqalculate < 2.0.0
Summary:
According to https://phabricator.kde.org/D7590 @filipesaraiva remove cln linking on commit 6ad659fc436a400756f3168ed3ca521b22195506.
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
Differential Revision: https://phabricator.kde.org/D11574