maintcheck: fix real and spurious warnings

maintcheck: fix real and spurious warnings

* lib/Makefile.am (install-data-hook): Correctly quote $(DESTDIR)
occurrences.
* Makefile.am (install-exec-hook, uninstall-hook): Likewise.
(autodiffs): Prefer '$(am__cd)' to plain 'cd', where warranted.
* doc/Makefile.am ($(srcdir)/amhello-1.0.tar.gz): Likewise.  Also,
prefer using AC_SUBST'd $(abs_top_builddir) over obtaining it at
runtime from $(top_builddir).
* syntax-check.mk (sc_mkinstalldirs): Tweak to whitelist known
harmless occurrences of the checked-against usages.
(sc_no_for_variable_in_macro): Likewise.
* tests/CheckListOfTests.am (maintainer-check-list-of-tests): Tweak
to avoid spuriously triggering the 'sc_no_for_variable_in_macro'
maintainer check.  Prefer '$(am__cd)' to plain 'cd' when warranted.