ECC-2280: Don't link libaec when ENABLE_AEC=OFF

[tomas-kral]

Description

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[joobog]

I think it makes sense to modernise this part a bit more. It was developed when libaec did not yet support CMake. If the solution in this branch works for you, I would prefer to use it:

https://github.com/ecmwf/eccodes/tree/bugfix/ecc-2280_libaec_linking

[tomas-kral]

Hi @joobog, thanks for looking at this! I'm happy whith whichever solution as long as it works as intended. :)