build: fix syntax check during 'make distcheck'

'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.

cfg.mk

@@ -85,7 +85,7 @@ local-checks-to-skip =			\
 ifeq ($(filter dist%, $(MAKECMDGOALS)), )
 local-checks-to-skip +=	sc_vulnerable_makefile_CVE-2012-3386
 else
-distdir: sc_vulnerable_makefile_CVE-2012-3386
+distdir: sc_vulnerable_makefile_CVE-2012-3386.z
 endif
 
 # Files that should never cause syntax check failures.