Gentoo ebuild doesn't list required USE flag
1) Install qtsingleapplication without X support
2) Attempt to install Musescore
3) Build fails with the same error as #52106: MuseScore git master fails to compile with -DUSE_SYSTEM_QTSINGLEAPPLICATION=ON
Admittedly, most people have Gentoo building with X flags by default, but without it qtsingleapplication only installs qtsinglecoreapplication.h and CMake can't find the directory with the given search. The workaround is a simple "Add
qtsingleapplication X to your package.use", but figuring out that's what's wrong took me a while. If instead the
>=dev-qt/qtsingleapplication-2.6.1_p20171024 line in the ebuild was replaced with
>=dev-qt/qtsingleapplication-2.6.1_p20171024[X], Portage would be able to lead those terminal-as-much-as-possible users like me directly to the solution before it even tries to install.
And, yes, I know the Gentoo packaging is lagging anyway. I just figured I'd point this out for whoever winds up drawing the update detail.