Output helpful message on "make test" when requirements are not met

Makefile

@@ -14,7 +14,10 @@ prepare:
 
 .PHONY: test
 test:
-	test -d os-autoinst
+	test -d os-autoinst || (echo "Missing test requirements, \
+link a local working copy of 'os-autoinst' into this \
+folder or call 'make prepare' to install download a copy necessary for \
+testing" && exit 2)
 	test -e tools || ln -s os-autoinst/tools .
 	tools/tidy --check
 	export PERL5LIB="../..:os-autoinst:lib:tests/installation:tests/x11:tests/qa_automation:$$PERL5LIB" ; for f in `find . -name \*.pm|grep -v /os-autoinst/` ; do perl -c $$f 2>&1 | grep -v " OK$$" && exit 2; done ; true