gnu: gtk+: Build GTK+ with its own xorg-server package.

This will allow us to update xorg-server directly on the master branch.

* gnu/packages/xorg.scm (xorg-server-1.19.2): New variable.
* gnu/packages/gtk.scm (gtk+) [native-inputs]: Use xorg-server-1.19.2 instead of
xorg-server.
[arguments]: Add xorg-server-1.19.2 to #:disallowed-references.

gnu/packages/gtk.scm

@@ -689,9 +689,12 @@ application suites.")
       ("pkg-config" ,pkg-config)
       ("gobject-introspection" ,gobject-introspection)
       ("python-wrapper" ,python-wrapper)
-      ("xorg-server" ,xorg-server)))
+      ;; By using a special xorg-server for GTK+'s tests, we reduce the impact
+      ;; of updating xorg-server directly on the master branch.
+      ("xorg-server" ,xorg-server-1.19.2)))
    (arguments
-    `(;; 47 MiB goes to "out" (24 of which is locale data!), and 26 MiB goes
+    `(#:disallowed-references (,xorg-server-1.19.2)
+      ;; 47 MiB goes to "out" (24 of which is locale data!), and 26 MiB goes
       ;; to "doc".
       #:configure-flags (list (string-append "--with-html-dir="
                                              (assoc-ref %outputs "doc")

gnu/packages/xorg.scm

@@ -5111,6 +5111,22 @@ communicates with the user via graphical controls such as buttons and
 draggable titlebars and borders.")
     (license license:x11)))
 
+;;; This package is intended to be used when building GTK+.
+(define-public xorg-server-1.19.2
+  (package
+    (inherit xorg-server)
+    (name "xorg-server")
+    (version "1.19.2")
+    (source
+      (origin
+        (method url-fetch)
+        (uri (string-append
+              "mirror://xorg/individual/xserver/"
+              name "-" version ".tar.bz2"))
+        (sha256
+         (base32
+          "1fw4b2lf75nsqkiyhn95b1c2if1l3cw5a188a1szx1d8l7sbk2jg"))))))
+
 (define-public xorg-server-xwayland
   (package
     (inherit xorg-server)