Support Saxon 10/11/12

[tomschr]

This (draft) PR fixes #702 and contains:

• an implementation in the autoconf/configure scripts to search for saxon scripts.
  At the moment, it prefers the script saxon12 (currently available in some home repos on OBS available, but not yet in Java:packages).
• configuration files for the each saxon scripts under etc/saxon. Probably not really needed, but it gives some flexibility to pass parameters to the XSLT processor.

It worked for me, but I'm not completely sure if this is how it should be. Some design decisions that need to be taken care of:

• How to let the user configure the saxon script to use? ATM, it uses the latest version (it prefers saxon12). Should that be configurable in the DAPS configuration?
• Should we install all Saxon config files? ATM only one is installed. For example, if saxon12 is found, it will install etc/saxon/saxon12.conf. I'm not completely sure if they are really needed, but I think we should have a way to use them (if really needed).
• How to adapt libexec/daps-xslt to support the many variants?
• Do we maybe need a general daps-saxon wrapper script?