Build via

  ./pre-inst-env guix build -K libxfce4ui

Check /tmp/guix-build-xfce4-embed-plugin-1.6.0.drv-*/*/config.log for libxfce4ui
and see what the surrounding lines say.

If that doesn't help,

  ./pre-inst-env guix environment --pure libxfce4ui

then

  pkg-config --cflags --libs libxfce4ui-1