bug#25590: Remove build number from emacs-version variable

[Eli Zaretskii <eliz@gnu.org>]

> From: Glenn Morris <rgm@gnu.org>
> Date: Tue, 31 Jan 2017 12:55:58 -0500
> 
> I'd like to suggest removing the build number from the variable emacs-version.
> Instead, it can go in a separate variable (eg emacs-build-number).
> The function emacs-version can still include it in parentheses, like it
> does for several other quantities.

If we do the above, we should make sure version-comparison functions
treat something like "25.2 (build 4)" correctly.

> Motivation:
> I think the build number is a largely internal detail that is only of
> interest to a small number of developers (as a data point, I've never
> used/wanted it for anything). Having it in the version string is
> confusing to people not familiar with Emacs development.

FWIW, I think this is a low-priority issue, as it most probably will
require quite a few changes, while the benefits are minor at best.

> (I am not aware of any other software that even has a concept of "build
> number".)

It's actually part of a version number (which many other projects
have, e.g., GDB just released version 7.12.1), except that we never
release such versions, they exist only on end-users' machines.  Other
than that, there's nothing in it which is special to Emacs.