Fix errors in distclean and maintainer-clean

Fix errors in distclean and maintainer-clean

The variable, LT_ARGZ_H, can be empty, which causes distclean and
maintainer-clean to error as a directory is attempted to be removed
instead of a file.

When running maintainer-clean for gnulib-tests, the .version file is
missing, which is needed by config.status, so the command will error.

* configure.ac: Update CONFIG_STATUS_DEPENDENCIES to use dotversion so
  that the .version file can be generated if needed.
* libltdl/ltdl.mk: Add conditional with LTARGZH_EXISTS to append path for
  LT_ARGZ_H file to BUILT_SOURCES.
* m4/ltdl.m4: Add AM_CONDITIONAL for LTARGZH_EXISTS that is set to
  true if LT_ARGZ_H is non-empty.
* NO-THANKS: Add 'Michael Pratt' for bug#73672 contribution.
* NEWS: List additional bug fix.