Add undocumented --version flag

Makefile

@@ -16,6 +16,14 @@ export GOPATH := $(CURDIR)/.gopath
 export PATH := $(PATH):$(CURDIR)/.gopath/bin
 GO_IMPORT_PATH := git.zx2c4.com/wireguard-go
 
+version.go:
+	@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
+
 .gopath/.created:
 	rm -rf .gopath
 	mkdir -p $(dir .gopath/src/$(GO_IMPORT_PATH))
@@ -27,7 +35,7 @@ vendor/.created: Gopkg.toml Gopkg.lock | .gopath/.created
 	cd .gopath/src/$(GO_IMPORT_PATH) && dep ensure -vendor-only -v
 	touch $@
 
-wireguard-go: $(wildcard *.go) $(wildcard */*.go) .gopath/.created vendor/.created
+wireguard-go: $(wildcard *.go) $(wildcard */*.go) .gopath/.created vendor/.created version.go
 	go build $(GO_BUILD_EXTRA_ARGS) -v $(GO_IMPORT_PATH)
 
 install: wireguard-go
@@ -40,4 +48,4 @@ update-dep: | .gopath/.created
 	command -v dep >/dev/null || go get -v github.com/golang/dep/cmd/dep
 	cd .gopath/src/$(GO_IMPORT_PATH) && dep ensure -update -v
 
-.PHONY: clean install update-dep
+.PHONY: clean install update-dep version.go

main.go

@@ -74,6 +74,11 @@ func warning() {
 }
 
 func main() {
+	if len(os.Args) == 2 && os.Args[1] == "--version" {
+		fmt.Printf("wireguard-go v%s\n\nUserspace WireGuard daemon for %s-%s.\nInformation available at https://www.wireguard.com.\nCopyright (C) Jason A. Donenfeld <Jason@zx2c4.com>.\n", WireGuardGoVersion, runtime.GOOS, runtime.GOARCH)
+		return
+	}
+
 	warning()
 
 	// parse arguments

version.go

@@ -0,0 +1,2 @@
+package main
+const WireGuardGoVersion = "0.0.20180519"