Allow more version number flexibility, by using Automake "gnu" mode.

Allow more version number flexibility, by using Automake "gnu" mode.

The "gnits" mode that PSPP has used for some time constrains version
numbers considerably.  For example, it prevents using two hyphens in the
version number to indicate, for instance, a prerelease and a Git commit
number like 0.8.0-pre1-g12345678.

This commit changes PSPP to use "gnu" mode instead, and then to also
enable the extra options that "gnits" mode enabled automatically
that we still want to use.

The Automake manual discourages using "gnits" mode anyhow:

     ...Unless you are a Gnits standards contributor, it is
     recommended that you avoid this option until such time as the Gnits
     standard is actually published (which may never happen).