You should try installing these dependencies:

cmake extra-cmake-modules kdecoration qt5-declarative qt5-x11extras kcoreaddons kcmutils kcolorscheme kconfig kguiaddons kiconthemes kwindowsystem

I'm not familiar with gentoo's packaging system, so I can't tell you what command should you use, but if you find a way to install it properly, you can help the project by writing a segment to the readme about the installing of dependencies in gentoo