maintcheck: reduce code duplication, increase coverage

maintcheck: reduce code duplication, increase coverage

* syntax-checks.mk: Move ...
(sc_tests_plain_aclocal,
sc_tests_plain_autoconf,
sc_tests_plain_perl,
sc_tests_plain_autoupdate,
sc_tests_plain_automake,
sc_tests_plain_make,
sc_tests_plain_autoheader,
sc_tests_plain_autoreconf,
sc_tests_plain_autom4te): ... all these targets ...
(sc_tests_plain_egrep_fgrep): ... and part of this target ...
(sc_tests_plain_check_rules): ... into this variable, and
reimplement their recipes ...
($(sc_tests_plain_check_rules)): ... with this static pattern
rule.  Enhance the coverage they offer a little since we are
at it.
(sc_m4_am_plain_egrep_fgrep): New rule, the part of the old
'sc_tests_plain_egrep_fgrep' rule that checked for use of
'fgrep' and 'egrep' in *.am and *.m4 fragments (rather than
in test scripts).
(syntax_check_rules): Update.
* t/dist-formats.tap: Fix bad use of "make" instead of "$MAKE"
revealed by the extra coverage.
* t/cond5.sh, t/auxdir-autodetect.sh: Cosmetic changes to avoid
spuriously triggering the maintainer checks due to the
enhanced coverage.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>