Update emacs: current release, use our x11 bits (#2052)

* Update emacs: current release, use our x11 bits

Add checksum for 25.1 release.

Rework the X support:

- use Spack's X11 bits
- add ability to specify an X toolkit (gtk or athena, default is gtk).
- change toolkit names to align with Emacs' configure usage.

* PEP8 cleanups.

* glib dependency should not be type=build

I'd like to blame that on a typo, but it's a few too many characters
for that to be viable.  I'm not sure what I was thinking.

* Pass X variant down: emacs->pango->cairo

* X variants default to False, warn on bad toolkit

Change the X variants for emacs, pango and cairo to default to False.

Check that the toolkit is a valid choice and give a reasonable error if
not.

* Fix flake8 issue, reword warning text

* gtkplus needs to use +X variant for pango to work

In order for a useful variant of pango to be built into the spec I
needed to make the dependency on gtkplus explicitly specify it's X
variant.  The X variant is the default, but that wasn't enough to make
it happy.  Since it's happiness is the most imporant thing in the
world, this change! :)

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

@@ -34,10 +34,14 @@ class Gtkplus(Package):
         '2.24.25', '612350704dd3aacb95355a4981930c6f',
         url="http://ftp.gnome.org/pub/gnome/sources/gtk+/2.24/gtk+-2.24.25.tar.xz")
 
+    variant('X', default=True, description="Enable an X toolkit")
+
     depends_on("atk")
     depends_on("gdk-pixbuf")
-    depends_on("pango")
     depends_on("glib")
+    depends_on("pango")
+    depends_on("pango~X", when='~X')
+    depends_on("pango+X", when='+X')
 
     def patch(self):
         # remove disable deprecated flag.