Adding some text to INSTALL

[Po Lu]

If nobody minds, I'll install the following text in INSTALL, above
"DETAILED BUILDING AND INSTALLATION":

* Alternative window systems

If you want to use Emacs on one of the alternative window systems
available on GNU/Linux and some Unix systems, such as Wayland or
Broadway, you can install the PGTK port of Emacs, which utilizes the
GTK+ toolkit to support them, by passing '--with-pgtk' to configure,
like this:

        ./configure --with-pgtk

Keep in mind that the only toolkit which will work with this option is
GTK+ version 3, and specifying any X-specific configuration option is an
error when it is enabled.

Afterwards, you will be able to switch between X, Wayland and Broadway
using the 'GDK_BACKEND' environment variable.  GTK+ should automatically
detect and use the correct value for your system, but you may have to
specify it manually.  For example, you must run Emacs like this to force
GTK+ to run under Broadway:

        GDK_BACKEND=broadway emacs ...

Where '...' denotes any further options you may want to pass to Emacs.

GNUstep also has support for the Wayland window system.  If that is what
you want, see nextstep/INSTALL.