Fix some bugs when using "$@" when there might be zero positional

Fix some bugs when using "$@" when there might be zero positional
arguments in cases where this matters.  Zsh doesn't support
the workaround ${1+"$@"} that has been used by Automake since
the Mon Dec  4 11:55:36 1995 change.

* aclocal.in: Use 'case' statement to work around problem.
* automake.in: Likewise.
* lib/ylwrap: Likewise.
* lib/missing: No need for workaround, as the command is never
invoked with zero arguments.
* tests/ccnoco.test: Likewise.
* missing: Update from lib/missing.