Clean up glibc version numbers in manual

[Paul Eggert]

On 02/21/2012 04:20 PM, Joseph S. Myers wrote:
> I will now propose that we ... generate 
> VERSION automatically so it is always the version number 

This is pretty standard in GNU projects, so much so that
there's an Automake feature supporting it (see VERSION in
<http://www.gnu.org/software/automake/manual/html_node/Texinfo.html>).
glibc doesn't use Automake of course, but it'd be reasonable for it
to use the same idea.