'make distcheck' was failing because a syntax check file,
.sc-start-sc_vulnerable_makefile_CVE-2012-3386, got left
behind.  I traced it to the 'distdir' rule depending on a
shortcut syntax-check name rather than the full rule name
normally used during 'local-check' from maint.mk.

* cfg.mk (distdir): Depend on full rule, not shorthand name.