Trying to build xfce for GTK3, exo's autogen.sh fails to find libxfce4ui-1.pc Removing the XDT_CHECK_PACKAGE([LIBXFCE4UI], [libxfce4ui-1], [4.12.0]) line allow the script to succeed.
I think it is required to pass --disable-gtk2 to configure.
You don't have libxfce4ui-1 (gtk2 version) installed ? Support for gtk2 in exo is now optionnal since https://git.xfce.org/xfce/exo/commit/?id=2097708c789a11b98e40a4aa49adc803613d57c0 , but is still listed in dependencies, it should be dropped (nothing use libxfce4ui-1 in exo master afaik).
It seems that exo/exo-2.pc also needs to be updated to Requires libxfce4ui-2 and not libxfce4ui-1 !
Romain Bouvier referenced this bugreport in commit 4bbf138fd9aeb819848c145518ced45a180b59a0 Bug #15199: Really allow libexo-1 to be disabled https://git.xfce.org/xfce/exo/commit?id=4bbf138fd9aeb819848c145518ced45a180b59a0