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.