more idiomatic makefile

Makefile

@@ -3,7 +3,7 @@
 
 # Target bodies live in script/ (Scripts to Rule Them All); the targets
 # below are thin shims that call them.
-.DEFAULT_GOAL := check
+.DEFAULT_GOAL := build
 
 VERSION ?= $(shell git describe --tags --always)
 
@@ -37,7 +37,9 @@ cibuild:
 hooks:
 	@script/install-precommit
 
-build:
+build: ./bin/rtnetmon
+
+./bin/rtnetmon: ./cmd ./internal go.sum go.mod
 	CGO_ENABLED=0 go build -trimpath -ldflags "-X main.Version=$(VERSION)" \
 		-o bin/rtnetmon ./cmd/rtnetmon