Musescore does not build

[polendri]

See the build failure on hydra. Relevant tail of the log:

Built target lrelease
build flags: -j4 -l4 SHELL=/nix/store/m2yxakijvp9db943qf8a0qbchyyg3dsn-bash-4.4-p12/bin/bash PREFIX=\$\(out\)
Scanning dependencies of target manpages
Scanning dependencies of target qtsingleapp_autogen
[  0%] Generating mscore.1.gz
make[2]: *** No rule to make target 'all.h', needed by 'thirdparty/singleapp/src/CMakeFiles/qtsingleapp_autogen'.  Stop.
make[1]: *** [CMakeFiles/Makefile2:8758: thirdparty/singleapp/src/CMakeFiles/qtsingleapp_autogen.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
Man pages have been compressed ready for installation.
Creating symlink alias for man pages.
Symlink alias: musescore.1.gz -> mscore.1.gz
[  0%] Built target manpages
make: *** [Makefile:152: all] Error 2
builder for ‘/nix/store/syvyc9csjsflr8bgr2rwbkb7zd6755hx-musescore-2.1.0.drv’ failed with exit code 2

Same thing happens to me when I build locally.

The last successful build was on 2509b62, and the first failure was on 422adc3 (a merge of staging branch), so it's something in between that busted it. I have no idea what's wrong, but hopefully it's obvious to someone else. I'd be able to leave a git bisect run running overnight to narrow down the issue otherwise.

@vandenoever

Edit: Digikam is also failing to build, so I suspect it's something common to the build chains of both.