On the 'staging' branch (commit
8a7a5dc7805f4628e60f90af6b2416f951d0c034), "calibre" fails to build:

g++ -c -pipe -O2 -Wall -Wextra -D_REENTRANT -fPIC -DQT_NO_DEBUG -DQT_PLUGIN -DQT_THEME_SUPPORT_LIB -DQT_FONTDATABASE_SUPPORT_LIB -DQT_SERVICE_SUPPORT_LIB -DQT_EVENTDISPATCHER_SUPPORT_LIB -DQT_GUI_LIB -DQT_DBUS_LIB -DQT_CORE_LIB -I. -isystem /gnu/store/3x2kak8abb6z2klch72kfff2qxzv00pj-libpng-1.6.37/include/libpng16 -isystem /gnu/store/haaam6v8l4s75mj9xmpb9gc78xk001y9-freetype-2.10.1/include/freetype2 -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5 -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtThemeSupport -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtThemeSupport/5.14.2 -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtThemeSupport/5.14.2/QtThemeSupport -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtFontDatabaseSupport -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtFontDatabaseSupport/5.14.2 -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtFontDatabaseSupport/5.14.2/QtFontDatabaseSupport -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtServiceSupport -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtServiceSupport/5.14.2 -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtServiceSupport/5.14.2/QtServiceSupport -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtEventDispatcherSupport -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtEventDispatcherSupport/5.14.2 -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtEventDispatcherSupport/5.14.2/QtEventDispatcherSupport -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtGui/5.14.2 -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtGui/5.14.2/QtGui -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtGui -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtCore/5.14.2 -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtCore/5.14.2/QtCore -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtDBus -isystem /gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/include/qt5/QtCore -I. -isystem /gnu/store/ka6amik38awpprjmbj01pnl9zsxgjjgz-libdrm-2.4.101/include/libdrm -I/gnu/store/59dq1n4wj3maa3kcar6hspj2zd82as7q-qtbase-5.14.2/lib/qt5/mkspecs/linux-g++ -o main.o /tmp/guix-build-calibre-3.42.0.drv-0/calibre-3.42.0/src/calibre/headless/main.cpp
/tmp/guix-build-calibre-3.42.0.drv-0/calibre-3.42.0/src/calibre/headless/headless_integration.cpp: In constructor ‘HeadlessIntegration::HeadlessIntegration(const QStringList&)’:
/tmp/guix-build-calibre-3.42.0.drv-0/calibre-3.42.0/src/calibre/headless/headless_integration.cpp:61:5: error: ‘screenAdded’ was not declared in this scope
     screenAdded(mPrimaryScreen);
     ^~~~~~~~~~~
make: *** [Makefile:336: headless_integration.o] Error 1

Presumably this can be fixed by updating to the latest version of
Calibre (4.17.0!).  Any takers?  :-)