Eliminate the separately maintained Lisp system version number.

  * Have `configure.ac' derive a version string which ASDF won't choke
    on and make that available as an output variable.

  * Move the ASD files to `.in' files, and substitute the source path
    into them at build time, so that they actually still work.

  * At build time, construct an `auto.lisp' file with the relevant version
    information; modify the ASD `.in' files to load this.

  * At install time, construct the ASD files differently, by stripping
    out the source path and substituting the version information bodily
    into the files.

  * Actually install the `sod-frontend' system definition, just in case
    it's useful to someone.

  * Actually remove the ASDF symlinks when we're finished.