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.