GTK+3: Provide updates to package dependencies.

WIP: I started trying to build gtkplus@3.20, but this package has many more
dependencies than v2 and it requires newer versions of existing packages. This
commit provides updates for 5 packages that are required by GTK+3. This is not
the complete set of changes required for GTK+3.

atk  - move default version from 2.14 -> 2.20.
glib - move default version from 2.42 -> 2.49
     - v2.49 requires pcre+utf as a new dependency.
pcre - if variant +utf is selected, add '--enable-unicode-properties' to the
       configure options.
libepoxy - new package to spack
         - manages OpenGL function pointers.
pango    - move default version from 1.36 -> 1.40

var/spack/repos/builtin/packages/libepoxy/package.py

@@ -0,0 +1,39 @@
+##############################################################################
+# Copyright (c) 2013-2016, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class Libepoxy(Package):
+    """Epoxy is a library for handling OpenGL function pointer management for
+    you."""
+    homepage = "https://github.com/anholt/libepoxy"
+    url      = "https://github.com/anholt/libepoxy/releases/download/v1.3.1/libepoxy-1.3.1.tar.bz2"
+
+    version('1.3.1', '96f6620a9b005a503e7b44b0b528287d')
+
+    def install(self, spec, prefix):
+        configure('--prefix={0}'.format(prefix))
+        make()
+        make('install')