Fix ‘make syntax-check’ complaints.

Fix ‘make syntax-check’ complaints.

* cfg.mk (local-checks-to-skip): Add sc_unportable_grep_q, which has
  too many false positives to bother with; for instance, it triggers
  on autoconf.texi’s discussion of why grep -q is unportable, and on
  the code in maint.mk that implements the check!
  (old_NEWS_hash): Update for commit b751bf49496ea3f0054533cfd63f977640abb07a,
  which fixed spelling errors in old NEWS.

* doc/autoconf.texi: Remove a doubled word.
* lib/autoconf/programs.m4: Remove a space immediately before a tab.
* lib/m4sugar/m4sh.m4 (_AS_IF): Rephrase documentation to avoid saying
  “if IF-FALSE” which triggers the prohibit_doubled_word check.