We weren't using the @FOO@ notation for a Makefile substitution,
but instead for a sed rule, so using [@]FOO@ instead avoids the
need to exempt this syntax check.

* cfg.mk (_makefile_at_at_check_exceptions): Delete.
* tools/Makefile.am (virt-xml-validate, virt-pki-validate): Avoid
tripping syntax-check.
Reported by Daniel P. Berrange.