libsecret's absence not detected by cmake

[fgaz]

If libsecret is not installed, it will not be detected until the linking failure at the end

Here's a log of this kind of failure: https://nix-cache.s3.amazonaws.com/log/pvx3aqxsbm1nyrjsg8h56169wkj4zr29-drawpile-2.1.17.drv