'zrepl version' and 'zrepl control version' subcommand + maintainer README

Version is autodetected on build using git
If it cannot be detected with git, an override must be provided.

For tracability of distros, the distroy packagers should override as
well, which is why I added a README entry for package mainatiners.

refs #35

Makefile

@@ -8,13 +8,23 @@ _TESTPKGS := $(ROOT) $(foreach p,$(SUBPKGS),$(ROOT)/$(p))
 
 ARTIFACTDIR := artifacts
 
+ifndef ZREPL_VERSION
+    ZREPL_VERSION := $(shell git describe --dirty 2>/dev/null || echo "ZREPL_BUILD_INVALID_VERSION" )
+    ifeq ($(ZREPL_VERSION),ZREPL_BUILD_INVALID_VERSION) # can't use .SHELLSTATUS because Debian Stretch is still on gmake 4.1
+        $(error cannot infer variable ZREPL_VERSION using git and variable is not overriden by make invocation)
+    endif
+endif
+GO_LDFLAGS := "-X github.com/zrepl/zrepl/cmd.zreplVersion=$(ZREPL_VERSION)"
+
+GO_BUILD := go build -ldflags $(GO_LDFLAGS)
+
 generate: #not part of the build, must do that manually
 	@for pkg in $(_TESTPKGS); do\
 		go generate "$$pkg" || exit 1; \
 	done;
 
 build:
-		go build -o $(ARTIFACTDIR)/zrepl
+		$(GO_BUILD) -o "$(ARTIFACTDIR)/zrepl"
 
 test:
 	@for pkg in $(_TESTPKGS); do \
@@ -54,10 +64,11 @@ docs-clean:
 		BUILDDIR=../artifacts/docs
 
 release-bins: $(ARTIFACTDIR) vet test
-	GOOS=linux GOARCH=amd64   go build -o "$(ARTIFACTDIR)/zrepl-linux-amd64"
-	GOOS=freebsd GOARCH=amd64 go build -o "$(ARTIFACTDIR)/zrepl-freebsd-amd64"
+	GOOS=linux GOARCH=amd64   $(GO_BUILD) -o "$(ARTIFACTDIR)/zrepl-linux-amd64"
+	GOOS=freebsd GOARCH=amd64 $(GO_BUILD) -o "$(ARTIFACTDIR)/zrepl-freebsd-amd64"
 
 release: release-bins docs
 
+
 clean: docs-clean
 	rm -rf "$(ARTIFACTDIR)"