Add git commit to the version output. (#900)

Now the output would be:
```
 ./coredns -version
CoreDNS-010
linux/amd64, go1.8, 241e3dbc
```
In case the local file has been modified, then the output would be (with --dirty):
```
 ./coredns -version
CoreDNS-010
linux/amd64, go1.8, 241e3dbc-dirty
```
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

Makefile

+GITCOMMIT     := $(shell git describe --dirty --always)
+
 BUILD_VERBOSE := -v
 
 TEST_VERBOSE := -v
@@ -8,7 +10,7 @@ all: coredns
 # TODO: Add .go file dependencies.
 .PHONY: coredns
 coredns: check godeps
-	CGO_ENABLED=0 go build $(BUILD_VERBOSE) -ldflags="-s -w"
+	CGO_ENABLED=0 go build $(BUILD_VERBOSE) -ldflags="-s -w -X github.com/coredns/coredns/coremain.gitCommit=$(GITCOMMIT)"
 
 .PHONY: deps
 deps: core/zmiddleware.go core/dnsserver/zdirectives.go godeps

Makefile.release

@@ -38,6 +38,7 @@ ARCH:=$(shell uname -m)
 GITHUB:=coredns
 DOCKER:=coredns
 DOCKER_IMAGE_NAME:=$(DOCKER)/$(NAME)
+GITCOMMIT:=$(shell git describe --dirty --always)
 
 all:
 	@echo Use the 'release' target to start a release
@@ -66,17 +67,17 @@ build: build-arm build-darwin build-linux
 .PHONY: build-linux
 build-linux:
 	@echo Building: linux $(VERSION)
-	mkdir -p build/Linux      && CGO_ENABLED=0 GOOS=linux             go build -ldflags="-s -w" -o build/Linux/$(NAME)
+	mkdir -p build/Linux      && CGO_ENABLED=0 GOOS=linux             go build -ldflags="-s -w -X github.com/coredns/coredns/coremain.gitCommit=$(GITCOMMIT)" -o build/Linux/$(NAME)
 
 .PHONY: build-darwin
 build-darwin:
 	@echo Building: darwin $(VERSION)
-	mkdir -p build/Darwin     && CGO_ENABLED=0 GOOS=darwin            go build -ldflags="-s -w" -o build/Darwin/$(NAME)
+	mkdir -p build/Darwin     && CGO_ENABLED=0 GOOS=darwin            go build -ldflags="-s -w -X github.com/coredns/coredns/coremain.gitCommit=$(GITCOMMIT)" -o build/Darwin/$(NAME)
 
 .PHONY: build-arm
 build-arm:
 	@echo Building: arm $(VERSION)
-	mkdir -p build/Linux/Arm  && CGO_ENABLED=0 GOOS=linux GOARCH=arm  go build -ldflags="-s -w" -o build/Linux/Arm/$(NAME)
+	mkdir -p build/Linux/Arm  && CGO_ENABLED=0 GOOS=linux GOARCH=arm  go build -ldflags="-s -w -X github.com/coredns/coredns/coremain.gitCommit=$(GITCOMMIT)" -o build/Linux/Arm/$(NAME)
 
 
 .PHONY: tar

coremain/run.go

@@ -168,7 +168,7 @@ func showVersion() {
 
 // versionString returns the CoreDNS version as a string.
 func versionString() string {
-	return fmt.Sprintf("%s-%s\n", caddy.AppName, caddy.AppVersion)
+	return fmt.Sprintf("%s-%s\n%s/%s, %s, %s\n", caddy.AppName, caddy.AppVersion, runtime.GOOS, runtime.GOARCH, runtime.Version(), gitCommit)
 }
 
 // setVersion figures out the version information