In deb packages, ensure libxtt version agrees with xtt version

[zanebeckwith]

The xtt tool package depends on the libxtt package. However, the xtt packaging doesn't require a specific version of the libxtt package, so those two versions can get mis-aligned.

[drbild]

I took a brief look at this on Friday.

We don't want to require an exact match in version, if a lower version will do. For example, if the changes in libxtt0 don't impact anything in xtt.

So the way we're packaging xtt is mostly correct. ${shlib:Depends} will try to set the dependency on libxtt0` to a good version.

However, we need to provide a shlibs or symbols file so that shlib:Depends can know which version to depend on.

For a C library like libxtt0, we should maintain a symbols file that indicates which versions added which symbols. If we make a backwards incompatible binary change, we should bump the SONAME number.

Notes: https://www.debian.org/doc/debian-policy/ch-sharedlibs.html#dependencies-between-the-library-and-other-packages