Allow overrides for GO and PROMU

This allows the user to override how/where promu and/or go is invoked.

Makefile

@@ -11,8 +11,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-GO    := GO15VENDOREXPERIMENT=1 go
-PROMU := $(GOPATH)/bin/promu
+GO    ?= GO15VENDOREXPERIMENT=1 go
+PROMU ?= $(GOPATH)/bin/promu
 pkgs   = $(shell $(GO) list ./... | grep -v /vendor/)
 
 PREFIX                  ?= $(shell pwd)
@@ -43,11 +43,11 @@ vet:
 	@echo ">> vetting code"
 	@$(GO) vet $(pkgs)
 
-build: promu
+build: $(PROMU)
 	@echo ">> building binaries"
 	@$(PROMU) build --prefix $(PREFIX)
 
-tarball: promu
+tarball: $(PROMU)
 	@echo ">> building release tarball"
 	@$(PROMU) tarball --prefix $(PREFIX) $(BIN_DIR)
 
@@ -55,10 +55,10 @@ docker:
 	@echo ">> building docker image"
 	@docker build -t "$(DOCKER_IMAGE_NAME):$(DOCKER_IMAGE_TAG)" .
 
-promu:
+$(GOPATH)/bin/promu promu:
 	@GOOS=$(shell uname -s | tr A-Z a-z) \
 		GOARCH=$(subst x86_64,amd64,$(patsubst i%86,386,$(shell uname -m))) \
 		$(GO) get -u github.com/prometheus/promu
 
 
-.PHONY: all style format build test test-e2e vet tarball docker promu
+.PHONY: all style format build test test-e2e vet tarball docker promu $(GOPATH)/bin/promu