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/pcre/package.py

@@ -29,7 +29,6 @@ class Pcre(Package):
     """The PCRE package contains Perl Compatible Regular Expression
        libraries. These are useful for implementing regular expression
        pattern matching using the same syntax and semantics as Perl 5."""
-
     homepage = "http://www.pcre.org"""
     url      = "ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/pcre-8.36.tar.bz2"
 
@@ -46,6 +45,7 @@ def install(self, spec, prefix):
         configure_args = ['--prefix=%s' % prefix]
         if '+utf' in spec:
             configure_args.append('--enable-utf')
+            configure_args.append('--enable-unicode-properties')
 
         configure(*configure_args)
         make()