cosmetic: more idiomatic way of figuring out directory

Makefile

@@ -15,10 +15,10 @@ WEAVEDNS_EXPORT=/var/tmp/weavedns.tar
 all: $(WEAVER_EXPORT) $(WEAVEDNS_EXPORT)
 
 update:
-	go get -u -f -v -tags -netgo ./$(shell dirname $(WEAVER_EXE)) ./$(shell dirname $(WEAVEDNS_EXE))
+	go get -u -f -v -tags -netgo ./$(dir $(WEAVER_EXE)) ./$(dir $(WEAVEDNS_EXE))
 
 $(WEAVER_EXE) $(WEAVEDNS_EXE):
-	go get -tags netgo ./$(shell dirname $@)
+	go get -tags netgo ./$(@D)
 	go build -ldflags "-extldflags \"-static\" -X main.version $(WEAVE_VERSION)" -tags netgo -o $@ ./$(shell dirname $@)
 	@strings $@ | grep cgo_stub\\\.go >/dev/null || { \
 		rm $@; \