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/gdk-pixbuf/package.py

@@ -36,6 +36,8 @@ class GdkPixbuf(Package):
 
     version('2.31.2', '6be6bbc4f356d4b79ab4226860ab8523')
 
+    depends_on("pkg-config", type="build")
+    depends_on("gettext")
     depends_on("glib")
     depends_on("jpeg")
     depends_on("libpng")