As per https://forums.gentoo.org/viewtopic-t-1095850-start-0.html dev-qt/linguist-tools needs dev-qt/qtwidgets[png]. Without it, the build fails owing to lack of a 'pushbutton' feature. Solution: add the missing dependency to the ebuild.
linguist-tools should not require qtwidgets[png] - in fact I can build it with qtwidgets removed completely. So, I suspect something else is not right. Could you please add the output of `qlist -I dev-qt -v`?
output of qlist -I dev-qt -v: dev-qt/designer-4.8.7 dev-qt/designer-5.12.3 dev-qt/linguist-tools-5.12.3 dev-qt/qdbus-5.12.3 dev-qt/qt3support-4.8.7 dev-qt/qtchooser-66 dev-qt/qtconcurrent-5.12.3 dev-qt/qtcore-5.12.3 dev-qt/qtdbus-4.8.7 dev-qt/qtdbus-5.12.3 dev-qt/qtdeclarative-4.8.7 dev-qt/qtdeclarative-5.12.3 dev-qt/qt-docs-5.12.3_p201904161302 dev-qt/qtgraphicaleffects-5.12.3 dev-qt/qtgui-4.8.7 dev-qt/qtgui-5.12.3-r1 dev-qt/qthelp-5.12.3 dev-qt/qtimageformats-5.12.3 dev-qt/qtlockedfile-2.4.1_p20171024 dev-qt/qtmultimedia-5.12.3 dev-qt/qtnetwork-5.12.3 dev-qt/qtnetworkauth-5.12.3 dev-qt/qtopengl-4.8.7 dev-qt/qtopengl-5.12.3 dev-qt/qtpaths-5.12.3 dev-qt/qtprintsupport-5.12.3 dev-qt/qtquickcontrols2-5.12.3 dev-qt/qtquickcontrols-5.12.3 dev-qt/qtscript-4.8.7 dev-qt/qtscript-5.12.3 dev-qt/qtsensors-5.12.3 dev-qt/qtsingleapplication-2.6.1_p20171024 dev-qt/qtspeech-5.12.3 dev-qt/qtsql-4.8.7-r1 dev-qt/qtsql-5.12.3 dev-qt/qtsvg-4.8.7 dev-qt/qtsvg-5.12.3 dev-qt/qttest-4.8.7 dev-qt/qttest-5.12.3 dev-qt/qttranslations-4.8.7 dev-qt/qtvirtualkeyboard-5.12.3 dev-qt/qtwayland-5.12.3 dev-qt/qtwebchannel-5.12.3 dev-qt/qtwebengine-5.12.3 dev-qt/qtwebkit-4.8.7 dev-qt/qtwebkit-5.212.0_pre20180120 dev-qt/qtwidgets-5.12.3 dev-qt/qtx11extras-5.12.3 dev-qt/qtxml-5.12.3 dev-qt/qtxmlpatterns-4.8.7 dev-qt/qtxmlpatterns-5.12.3
I suspect it's from having an old version of qtwidgets installed, the src.pro configuration file looks like it's doing autodetection of the existence of qtwidgets and its png support, but assumes it's the current version. Then the misordering means that it tries to compile against an old version of qtwidgets+png and fails.