Do not build if nothing to do

Makefile

@@ -1,26 +1,33 @@
 PREFIX ?= /usr
 DESTDIR ?=
 BINDIR ?= $(PREFIX)/bin
+export GOPATH ?= $(CURDIR)/.gopath
+
+default: generate-version-and-build
 
-ifeq ($(shell go env GOOS),linux)
-ifeq ($(wildcard .git),)
+ifeq ($(shell go env GOOS)|$(wildcard .git),linux|)
 $(error Do not build this for Linux. Instead use the Linux kernel module. See wireguard.com/install/ for more info.)
 else
-$(shell printf 'package main\nconst UseTheKernelModuleInstead = 0xdeadbabe\n' > ireallywantobuildon_linux.go)
-endif
+ireallywantobuildon_linux.go:
+	@printf "WARNING: This software is meant for use on non-Linux\nsystems. For Linux, please use the kernel module\ninstead. See wireguard.com/install/ for more info.\n\n" >&2
+	@printf 'package main\nconst UseTheKernelModuleInstead = 0xdeadbabe\n' > "$@"
+clean-ireallywantobuildon_linux.go:
+	@rm -f ireallywantobuildon_linux.go
+.PHONY: clean-ireallywantobuildon_linux.go
+clean: clean-ireallywantobuildon_linux.go
+wireguard-go: ireallywantobuildon_linux.go
 endif
 
-export GOPATH ?= $(CURDIR)/.gopath
-
-all: wireguard-go
+MAKEFLAGS += --no-print-directory
 
-version.go:
+generate-version-and-build:
 	@export GIT_CEILING_DIRECTORIES="$(realpath $(CURDIR)/..)" && \
 	tag="$$(git describe --dirty 2>/dev/null)" && \
 	ver="$$(printf 'package main\nconst WireGuardGoVersion = "%s"\n' "$$tag")" && \
-	[ "$$(cat $@ 2>/dev/null)" != "$$ver" ] && \
-	echo "$$ver" > $@ && \
-	git update-index --assume-unchanged $@ || true
+	[ "$$(cat version.go 2>/dev/null)" != "$$ver" ] && \
+	echo "$$ver" > version.go && \
+	git update-index --assume-unchanged version.go || true
+	@$(MAKE) wireguard-go
 
 wireguard-go: $(wildcard *.go) $(wildcard */*.go)
 	go build -v -o "$@"
@@ -31,4 +38,4 @@ install: wireguard-go
 clean:
 	rm -f wireguard-go
 
-.PHONY: clean install version.go
+.PHONY: default clean install generate-version-and-build