It seems that some changes in the way lilypond-books output is stored have broken certain build rules, particularly ones that clean stuff. For example,
$ make check $ make test-clean $ time make check real 0m31.410s
user 0m25.598s sys 0m4.084s $ make test-clean && rm -fr out/lybook-db $ time make check real 6m42.076s user 6m18.948s sys 0m14.981s
Since the first build time is so short, it seems that 'make test-clean' is not correctly cleaning everything. I think there is a similar problem with make web-clean, but I haven't investigated thoroughly because 'make web' takes so long anyway.
Also, 'make check' prints an informative message: For tracking crashes: use