makefile: Permit check-spec-links job on SDK subsets

Author: rpavlik

specification/Makefile

@@ -446,9 +446,9 @@ CHECK_SPEC_LINKS_SCRIPT = $(CURDIR)/scripts/check_spec_links.py --ignore_count=0
 # Would like those in the logs, but not in the count for build-breaking.
 
 check-spec-links:
-	$(QUIET)$(CHECK_MARKUP_SCRIPT) -Werror
-	$(QUIET)$(PYTHON) $(CHECK_SPEC_LINKS_SCRIPT)
-	$(QUIET)env FAIL_IF_COULD_NOT_VALIDATE=false ./checkXml.sh
+	$(QUIET)if [ -f $(CHECK_MARKUP_SCRIPT) ] && [ -f $(SPECSRC) ]; then $(CHECK_MARKUP_SCRIPT) -Werror; fi
+	$(QUIET)if [ -f $(SPECSRC) ]; then $(PYTHON) $(CHECK_SPEC_LINKS_SCRIPT); fi
+	$(QUIET)if [ -f checkXml.sh ] && [ -f registry/registry.rnc ]; then env FAIL_IF_COULD_NOT_VALIDATE=false ./checkXml.sh; fi
 	$(QUIET)$(PYTHON) $(SCRIPTS)/xml_consistency.py
 
 .PHONY: check-spec-links