xwayland: remove -Dxvfb=false, and optionally install Xvfb

With the default (-Dxvfb=true), Xvfb is built but not installed.  So
leave the default (as Xvfb may be used in test), and add instruction to
install it if the user won't use xorg-server.

x/installing/xwayland.xml

@@ -119,7 +119,6 @@ mkdir build &&
 cd    build &&
 
 meson --prefix=$XORG_PREFIX         \
-      -Dxvfb=false                  \
       -Dxkb_output_dir=/var/lib/xkb \
       ..                            &&
 ninja</userinput></screen>
@@ -134,7 +133,7 @@ ninja</userinput></screen>
       url="https://gitlab.freedesktop.org/xorg/xserver/-/blob/xwayland-21.1/.gitlab-ci/debian-install.sh">
       Upstream continuous integration build</ulink>.
     </para>
-
+<!--
     <para>
       Furthermore, an X server needs to be running during the build of
       <application>xts</application>. If not running the tests in a
@@ -142,7 +141,7 @@ ninja</userinput></screen>
       by removing the <parameter>-Dxvfb=false</parameter> above. The
       instructions below expect this has been done.
     </para>
-
+-->
     <para>
       Running the tests involves downloading two other frameworks, in addition
       to the mentioned optional dependencies:
@@ -189,6 +188,14 @@ EOF</userinput></screen>
 <screen role="root" revision="systemd"><userinput>ninja install &amp;&amp;
 mkdir -pv /etc/X11/xorg.conf.d</userinput></screen>
 
+    <para>
+      If <xref linkend='xorg-server'/> is not installed and you do not plan
+      to install it later, you can install <command>Xvfb</command> from this
+      package.  As the &root; user:
+    </para>
+
+<screen role="nodump"><userinput>install -vm755 hw/vfb/Xvfb /usr/bin</userinput></screen>
+
   </sect2>
 
   <sect2 role="commands">
@@ -199,14 +206,14 @@ mkdir -pv /etc/X11/xorg.conf.d</userinput></screen>
       installing a manual page for <command>Xserver</command>,
       which is not part of this package.
     </para>
-
+<!--
     <para>
       <parameter>-Dxvfb=false</parameter>: Prevents building the
       <command>xvfb</command> program, which is also installed by
       <xref linkend="xorg-server"/>. Remove this option if you do not plan to
       install the X server.
     </para>
-
+-->
     <para>
       <command>mkdir -pv /etc/X11/xorg.conf.d</command>: This directory is
       needed to put configuration files for Xwayland. This command ensures