Cleanup initialization of configure process (issue 549350043 by jonas.hahnfeld@gmail.com)

[dak]

https://codereview.appspot.com/549350043/diff/581410043/configure.ac
File configure.ac (right):

https://codereview.appspot.com/549350043/diff/581410043/configure.ac#newcode7
configure.ac:7: AC_INIT([LilyPond], [2.21.0], [bug-lilyp...@gnu.org],
[lilypond], [http://lilypond.org/])
Hardwiring the version number in this manner is a real maintenance drag.
 Couldn't autogen.sh create a VERSION.AC file (or something like it)
containing only the version number that is just included here?

https://codereview.appspot.com/549350043/