Makefile: rely on GNU make; derive syntax-check rule names

Makefile: rely on GNU make; derive syntax-check rule names

Rather than requiring that each sc_ rule be listed as a dependent
of "check", use features of GNU make to derive the list.
* Makefile (syntax-check-rules): Define.
(check): Depend on the new variable, not the hard-coded list.
Add a comment listing a few requirements.