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/pango/package.py

@@ -24,6 +24,7 @@
 ##############################################################################
 from spack import *
 
+
 class Pango(Package):
     """Pango is a library for laying out and rendering of text, with
        an emphasis on internationalization. It can be used anywhere
@@ -33,6 +34,7 @@ class Pango(Package):
     url      = "http://ftp.gnome.org/pub/gnome/sources/pango/1.36/pango-1.36.8.tar.xz"
 
     version('1.36.8', '217a9a753006275215fa9fa127760ece')
+    version('1.40.1', '6fc88c6529890d6c8e03074d57a3eceb')
 
     depends_on("harfbuzz")
     depends_on("cairo")