check vendor directory

This runs "dep check" to verify that the vendor directory is
up-to-date and meets expectations (= done with dep >= 0.5.0).

build.make

@@ -117,6 +117,16 @@ test-fmt:
 		false; \
 	fi
 
+# This test only runs when dep >= 0.5 is installed, which is the case for the CI setup.
+.PHONY: test-vendor
+test: test-vendor
+test-vendor:
+	@ echo; echo "### $@:"
+	@ case "$$(dep version 2>/dev/null | grep 'version *:')" in \
+		*v0.[56789]*) dep check && echo "vendor up-to-date" || false;; \
+		*) echo "skipping check, dep >= 0.5 required";; \
+	esac
+
 .PHONY: test-subtree
 test: test-subtree
 test-subtree: