* INSTALL (Alternative window systems): Tell those using exclusively X
not to use the PGTK port.
to specify any other X-specific configuration option when PGTK is
enabled.
+If you use exclusively X, do not use the PGTK port. There are a
+number of respects in which the regular --with-x-toolkit=gtk build
+works better. The PGTK port should not be considered a simple upgrade
+from --with-x-toolkit=gtk.
+
With the PGTK build, you will be able to switch between running Emacs
on X, Wayland and Broadway using the 'GDK_BACKEND' environment
variable. GTK+ should automatically detect and use the correct value