gnu: libx11: Update to 1.6.12 [fixes CVE-2020-14363].

* gnu/packages/xorg.scm (libx11/fixed): New variable.
(libx11)[replacement]: Graft.

gnu/packages/xorg.scm

@@ -5468,6 +5468,7 @@ (define-public libx11
         (sha256
           (base32
             "09k2pqmqbn2m1bpgl7jfxyqxaaxsnzbnp2bp8ycmqldqi5ln4j5g"))))
+    (replacement libx11/fixed)
     (build-system gnu-build-system)
     (outputs '("out"
                "doc"))                            ;8 MiB of man pages + XML
@@ -5495,6 +5496,21 @@ (define-public libx11
     (description "Xorg Core X11 protocol client library.")
     (license license:x11)))
 
+(define-public libx11/fixed
+  (package
+    (inherit libx11)
+    (version "1.6.12")
+    (source
+      (origin
+        (method url-fetch)
+        (uri (string-append
+               "mirror://xorg/individual/lib/libX11-"
+               version
+               ".tar.bz2"))
+        (sha256
+          (base32
+            "1ivfzl1qwk8zh7gc0m5vb58gdxz11jwg7w3d356w16j1d5s2427i"))))))
+
 ;; packages of height 5 in the propagated-inputs tree
 
 (define-public libxcursor