automake: be robust against directories containing ().

automake: be robust against directories containing ().

This change fixes https://bugs.gnu.org/14196.

* m4/missing.m4 (AM_MISSING_HAS_RUN): always quote the
invocation (not just if $am_aux_dir contains space or tab), in
case $am_aux_dir contains () or other metachars not rejected by
AM_SANITY_CHECK; quoting with '...' suggested by Jim Meyering.
* t/man6.sh (HELP2MAN): adjust grep since missing value
is quoted now.
* t/am-missing-prog.sh: likewise.