Rewrite maintainer-check in separate tests, parallelizable.

Rewrite maintainer-check in separate tests, parallelizable.

* Makefile.am (sc_diff_automake_in_automake, sc_perl_syntax)
(sc_no_brace_variable_expansions, sc_rm_minus_f)
(sc_no_for_variable_in_macro, sc_mkinstalldirs)
(sc_pre_normal_post_install_uninstall, sc_perl_no_undef)
(sc_perl_no_split_regex_space, sc_cd_in_backquotes)
(sc_cd_relative_dir, sc_perl_at_uscore_in_scalar_context)
(sc_perl_local_no_parens, sc_perl_local)
(sc_AMDEP_TRUE_in_automake_in, sc_tests_make_without_am_makeflags)
(sc_tests_plain_make, sc_tests_plain_autoconf)
(sc_tests_plain_autoupdate, sc_tests_plain_automake)
(sc_tests_here_document_format, sc_tests_Exit_not_exit)
(sc_tests_automake_fails, sc_tests_plain_aclocal)
(sc_tests_plain_perl, sc_tests_required_after_defs)
(sc_tests_overriding_macros_on_cmdline, sc_tests_plain_sleep)
(sc_tests_plain_egrep_fgrep, sc_mkdir_p, sc_perl_at_substs)
(sc_unquoted_DESTDIR, sc_tabs_in_texi, sc_at_in_texi): New rules,
all phony, all listed in ...
(syntax_check_rules): ... this new variable and split out from ...
(maintainer-check): ... this rule.  Depend on $(syntax_check_rules).

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>