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/atk/package.py

@@ -32,6 +32,7 @@ class Atk(Package):
     homepage = "https://developer.gnome.org/atk/"
     url      = "http://ftp.gnome.org/pub/gnome/sources/atk/2.14/atk-2.14.0.tar.xz"
 
+    version('2.20.0', '5187b0972f4d3905f285540b31395e20')
     version('2.14.0', 'ecb7ca8469a5650581b1227d78051b8b')
 
     depends_on("glib")