Under the current nvidia-drivers ebuild the following header files and lib are not installed

/usr/include/NVCtrl
nv_control.h
NVCtrlLib.h
NVCtrl.h
/usr/lib64/libXNVCtrl.a

Using Gentoo nvidia ebuild (same ver) the headers etc are installed

From the gentoo ebuild:
if use static-libs; then
dolib.a "${S}"/nvidia-settings-${PV}/src/libXNVCtrl/libXNVCtrl.a
insinto /usr/include/NVCtrl
doins "${S}"/nvidia-settings-${PV}/src/libXNVCtrl/*.h
fi

Without these files, xfce-extra/xfce4-sensors-plugin will not compile.

The Funtoo ebuild is so different to Gentoo and I am not sure why

nvidia-settings (tools) does not look like it's being built in the Funtoo ebuild also