Built, installed, worked like a charm in gwenview. But not with Digikam. Someone who's struggling with this may benefit. Skimmed settings interface and here is the answer (It is not consistent with other Qt apps to require jxl to be enabled manually. What is the reasoning behind this, I wonder):
Settings -> Configure digikam -> Views -> Mime Types, add "jxl" (without brackets) to the Additional image file extensions field.
Now the jxl thumbnails should be displayed properly (you can try to add the jpegxl-logo.jxl from testfiles to digikam to test this right away).
Built, installed, worked like a charm in gwenview. But not with Digikam. Someone who's struggling with this may benefit. Skimmed settings interface and here is the answer (It is not consistent with other Qt apps to require jxl to be enabled manually. What is the reasoning behind this, I wonder):
Settings -> Configure digikam -> Views -> Mime Types, add "jxl" (without brackets) to the Additional image file extensions field. Now the jxl thumbnails should be displayed properly (you can try to add the jpegxl-logo.jxl from testfiles to digikam to test this right away).