Isolate variables set by the configuration script

This is useful to developpers as it reduces the number of ./configure runs
required when working on the sources.
Save installed version of the package
Update for newer version of autoconf

It seems that newer versions of autoconf (e.g. 2.69) divert away everything
before AC_INIT. We therefore move version management after AC_INIT.
Handle architecture, configuration and package variables
Prevent multiple inclusion of ancillary files
Simplify file and update comments
Rework the cleaning framework

- Do not define cleaning variables with empty values
- Add documentation
- Add a debugging procedure
Rework installation routines

- Add documention
- Add pseudo commands INSTALL_*
- Add pseudo commands COPYTRESS_*
- Add display-files to print debug information