tests: AC_CONFIG_MACRO_DIRS: ignore inevitable failures with old autoconf

tests: AC_CONFIG_MACRO_DIRS: ignore inevitable failures with old autoconf

When "older" version of autoconf are used (that is, those before commit
v2.69-44-g1ed0548), we have no sane way to prevent the autom4te invocation
issued from aclocal to possibly display warnings "MACRO m4_require'd but
not m4_defun'd".  That's not a big deal, because that just means that
people using pre-2.70 autoconf with cutting-edge automake will see few
spurious warnings, but the actual semantics will remain correct.

However, this blemish was causing a couple of annoying testsuite failures.
Solve this by simply skipping the affected tests when older (pre-2.70)
autoconf versions are used.

* t/aclocl-macrodir.tap ("AC_CONFIG_MACRO_DIR interaction with
AC_REQUIRE"): Skip when older autoconf is in use.
* t/aclocl-macrodirs.tap ("AC_CONFIG_MACRO_DIR interaction with
AC_REQUIRE"): Likewise.

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