libdecor_decorate() already creates an xdg_surface and xdg_toplevel
internally for the frame. Calling xdg_surface_get_toplevel() again on
that same xdg_surface just to fetch the toplevel for icon handling
assigns the xdg_toplevel role to the underlying wl_surface a second
time, which is a protocol violation.
On GNOME Shell 50 (mutter) this is no longer tolerated: mutter
terminates the Wayland connection ("WL: error in client
communication"), and since libdecor_shellInit() waits for the initial
configure with a blocking, error-less libdecor_dispatch(-1) loop, the
client hangs forever with no window and no diagnostic output. This
made -DENABLE_LIBDECOR=ON (the default when libdecor-0 is present)
appear completely broken on current GNOME/Wayland.
Use libdecor_frame_get_xdg_toplevel() instead, which returns the
xdg_toplevel libdecor already owns, without requesting a second role
assignment.
Require libdecor-0 >= 0.1.1, which is available in Debian oldstable,
so the new accessor is guaranteed by the build dependency.
This allows us to build with libdecor enabled as the selection to use it
is decided upon at runtime if the compositor `gnome-shell` is detected.
If the libdecor development headers are installed, by default it will
now be compiled in unless overridden by the user at compile time.
