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. Add AC_SUBST for
  LT_DOLLAR_SIGN use in libltdl/ltdl.mk.
* libltdl/ltdl.mk: Add conditional to check if LT_ARGZ_H is empty.