Created attachment 509936 [details] build log of calibre-2.78.0 Because of the profile update to 17.0 I rebuilt all my packages, calibre-2.78 is one of the build failures. I also run the still keyworded dev-qt/qtcore-5.9.3 and as far as I remember I never rebuilt calibre after upgrading to qt-5.9. So 2 likely causes are either the new profile and compiler (gcc-6.4 instead of 5) or the qt version.
Created attachment 509938 [details] emerge --info
app-text/calibre-3.8.0 builds fine on my system with qt-5.9.3
Confirmed issue on Qt 5.9 with upstream https://bugs.launchpad.net/calibre/+bug/1695535 Their solution is to apply their patch or upgrade.
Since you've keyworded qt-5.9, you also need to keyword the latest calibre and dependencies. See the Package list field in bug 642796.
Fixed in app-text/calibre-3.15.0.