New windows builds

[Phillip Lord]

I've made some changes to the Emacs windows build packaging. I've pushed
a new snapshot build to
https://alpha.gnu.org/gnu/emacs/pretest/windows/.

Comments are welcome (specifically on the packaging!).

Background:

As Michael has already noticed, I've updated Emacs-26 with the build
scripts that I am now using to make windows builds. This seemed like a
good idea to me; the process has evolved over the the Emacs-25 cycle but
this wasn't reflected in the Emacs repository. This seemed like a bad
idea, as there were a couple of packaging related bugs.

The new scripts automate both the build of the Emacs zip files
themselves and the "dependencies" binaries and source zip files. I've
added one new feature (already mentioned) which is the ability to build
a snapshot from an arbitrary branch.

Phil