x.org: mirror mixin

According to my nightly CI/CD tests, x.org is another large provider
of software in common build chains that is often down.

Added a hand-selected amount of mirrors that is well up-to-sync.
Tested with `util-macros` that has a quite "recent" patch release.

Other packages to follow in an individual PR.

lib/spack/spack/build_systems/xorg.py

@@ -0,0 +1,37 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+import spack.util.url
+import spack.package
+
+
+class XorgPackage(spack.package.PackageBase):
+    """Mixin that takes care of setting url and mirrors for x.org
+       packages."""
+    #: Path of the package in a x.org mirror
+    xorg_mirror_path = None
+
+    #: List of x.org mirrors used by Spack
+    base_mirrors = [
+        'https://www.x.org/archive/individual/',
+        'https://mirrors.ircam.fr/pub/x.org/individual/',
+        'http://xorg.mirrors.pair.com/individual/'
+    ]
+
+    @property
+    def urls(self):
+        self._ensure_xorg_mirror_path_is_set_or_raise()
+        return [
+            spack.util.url.join(m, self.xorg_mirror_path,
+                                resolve_href=True)
+            for m in self.base_mirrors
+        ]
+
+    def _ensure_xorg_mirror_path_is_set_or_raise(self):
+        if self.xorg_mirror_path is None:
+            cls_name = type(self).__name__
+            msg = ('{0} must define a `xorg_mirror_path` attribute'
+                   ' [none defined]')
+            raise AttributeError(msg.format(cls_name))

lib/spack/spack/pkgkit.py

@@ -32,6 +32,7 @@
 from spack.build_systems.sip import SIPPackage
 from spack.build_systems.gnu import GNUMirrorPackage
 from spack.build_systems.sourceware import SourcewarePackage
+from spack.build_systems.xorg import XorgPackage
 
 from spack.mixins import filter_compiler_wrappers