Fix for cpp17 : Ld couldnt reach references when we use constexpr

/bin/ld: CMakeFiles/gtkconfig.dir/configvalueprovider.cpp.o: in function ConfigValueProvider::windowDecorationsButtonsImages() const': /home/kde-dev/kde/src/kde-gtk-config/kded/configvalueprovider.cpp:205: undefined reference to DecorationPainter::ButtonGeometry' /bin/ld: /bin/ld: DWARF error: could not find variable specification at offset 28298 kwin_bridge/libKWinBridge.a(auroraedecorationpainter.cpp.o): in function AuroraeDecorationPainter::paintButton(QPainter&, QString const&, QString const&) const': /home/kde-dev/kde/src/kde-gtk-config/kded/kwin_bridge/auroraedecorationpainter.cpp:29: undefined reference to DecorationPainter::ButtonGeometry' /bin/ld: kwin_bridge/libKWinBridge.a(dummydecorationbridge.cpp.o): in function KDecoration2::DummyDecorationBridge::paintButton(QPainter&, QString const&, QString const&)': /home/kde-dev/kde/src/kde-gtk-config/kded/kwin_bridge/dummydecorationbridge.cpp:107: undefined reference to DecorationPainter::ButtonGeometry' /bin/ld: /home/kde-dev/kde/src/kde-gtk-config/kded/kwin_bridge/dummydecorationbridge.cpp:135: undefined reference to DecorationPainter::ButtonGeometry' /bin/ld: kwin_bridge/libKWinBridge.a(dummydecorationbridge.cpp.o): in function KDecoration2::DummyDecorationBridge::passMouseHoverEventToButton(KDecoration2::DecorationButton*) const': /home/kde-dev/kde/src/kde-gtk-config/kded/kwin_bridge/dummydecorationbridge.cpp:177: undefined reference to DecorationPainter::ButtonGeometry' /bin/ld: kwin_bridge/libKWinBridge.a(dummydecorationbridge.cpp.o):/home/kde-dev/kde/src/kde-gtk-config/kded/kwin_bridge/dummydecorationbridge.cpp:178: more undefined references to DecorationPainter::ButtonGeometry' follow collect2: error: ld returned 1 exit status

after patch it compiles without any problem