syntax-check: fix violations and re-enable sc_makefile_at_at_check.

syntax-check: fix violations and re-enable sc_makefile_at_at_check.

At some point we were supporting some undetermined `broken
make', as evidenced by having carried the following code since
2003:
  ## use @LIBLTDL@ because some broken makes do not accept
  ## macros in targets, we can only do this because our LIBLTDL
  ## does not contain $(top_builddir).
  @LIBLTDL@: $(top_distdir)/libtool \
  ...
However, we've also had *many* cases of macros in targets for
just as long, so most likely we never fully supported makes
allegedly broken in this way.  As of this release, we explicitly
no longer support make implementations that do not accept macros
in targets.
* cfg.mk (local-checks-to-fix): Remove sc_makefile_at_at_check
from list of disabled checks.
* configure.ac (order-only prerequisites): Test with the
order-only pipe symbol in a macro.
* Makefile.am, tests/mdemo/Makefile.am: Convert all @FOO@ to
$(FOO).

Signed-off-by: Gary V. Vaughan <gary@gnu.org>