gcr3: Disable ssh agent

Let gcr4 provide it, so they don't overwrite each other.

It looks like gcr-ssh-agent is completely useless on non-GNOME desktop
environments, and GNOME users will build gcr4 anyway.

gnome/platform/gcr.xml

@@ -129,6 +129,7 @@ cd    build &&
 meson setup --prefix=/usr       \
             --buildtype=release \
             -Dgtk_doc=false     \
+            -Dssh_agent=false   \
             ..                  &&
 ninja</userinput></screen>
 
@@ -181,6 +182,13 @@ ninja</userinput></screen>
     </para>
     -->
 
+    <para>
+      <parameter>-Dssh_agent=false</parameter>: Prevent a conflict with
+      <xref linkend='gcr4'/>.  Omit this switch if you do not plan to
+      install <xref linkend='gcr4'/> and you want to use
+      <command>gcr-ssh-agent</command>.
+    </para>
+
     <para>
       <option>-Dgtk=false</option>: Use this switch if you
       haven't installed <xref linkend="gtk3"/>. Note that