There seem to be some enchancements with what comes delivered with Fedora 32 (not yet released). I just wanted to drop this issue here so it can be investigated and fixed. Fedora 31 of course works perfectly with compiling xfce4-panel 4.15.x.

make[3]: Leaving directory '/root/rpmbuild/BUILD/xfce4-panel-4.15.1/libxfce4panel'
make[3]: Entering directory '/root/rpmbuild/BUILD/xfce4-panel-4.15.1/libxfce4panel'
/usr/bin/g-ir-compiler --includedir=. --includedir=/usr/share/gir-1.0 --includedir=. --includedir=. libxfce4panel-2.0.gir -o libxfce4panel-2.0.typelib
make[3]: Leaving directory '/root/rpmbuild/BUILD/xfce4-panel-4.15.1/libxfce4panel'
make[3]: Entering directory '/root/rpmbuild/BUILD/xfce4-panel-4.15.1/libxfce4panel'
/usr/bin/vapigen-0.48  --library libxfce4panel-2.0 --metadatadir .  --girdir /usr/share/gir-1.0  libxfce4panel-2.0.gir
warning: empty metadata
libxfce4panel-2.0.gir:2234.7-2234.40: error: overriding method `Xfce.PanelPlugin.remote_event' is incompatible with base method `bool Xfce.PanelPluginProvider.remote_event (string, GLib.Value, uint)': too few parameters.
      <glib:signal name="remote-event" when="last">
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Generation failed: 1 error(s), 1 warning(s)
make[3]: *** [/usr/share/vala/Makefile.vapigen:61: libxfce4panel-2.0.vapi] Error 1
make[3]: Leaving directory '/root/rpmbuild/BUILD/xfce4-panel-4.15.1/libxfce4panel'
make[2]: *** [Makefile:789: all] Error 2
make[1]: *** [Makefile:607: all-recursive] Error 1
make: *** [Makefile:518: all] Error 2