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/glib/package.py

@@ -32,14 +32,16 @@ class Glib(Package):
        and interfaces for such runtime functionality as an event loop,
        threads, dynamic loading and an object system."""
     homepage = "https://developer.gnome.org/glib/"
-    url = "http://ftp.gnome.org/pub/gnome/sources/glib/2.42/glib-2.42.1.tar.xz"
+    url      = "http://ftp.gnome.org/pub/gnome/sources/glib/2.42/glib-2.42.1.tar.xz"
 
+    version('2.49.4', 'e2c87c03017b0cd02c4c73274b92b148')
     version('2.42.1', '89c4119e50e767d3532158605ee9121a')
 
-    depends_on("libffi")
-    depends_on("zlib")
-    depends_on("pkg-config", type='build')
+    depends_on('libffi')
+    depends_on('zlib')
+    depends_on('pkg-config', type='build')
     depends_on('gettext', when=sys.platform == 'darwin')
+    depends_on('pcre+utf', when='@2.49:')
 
     # The following patch is needed for gcc-6.1
     patch('g_date_strftime.patch')