Allow overriding url to fetch go from by env var

If you want to use a different binary package than the officially
provided, you now can point the env var GOURL to the package you
want to get instead.

Change-Id: I1cefe2998bc86435cfbd058ba398a7b6c4e7d031

Makefile

@@ -38,7 +38,7 @@ tarball: build
 	tar -C $(BUILD_PATH)/package -czf prometheus.tar.gz .
 
 $(BUILD_PATH)/cache/$(GOPKG):
-	curl -o $@ http://go.googlecode.com/files/$(GOPKG)
+	curl -o $@ $(GOURL)/$(GOPKG)
 
 clean:
 	$(MAKE) -C $(BUILD_PATH) clean

Makefile.INCLUDE

@@ -30,17 +30,18 @@ ARCH=$(shell uname -m)
 
 BUILD_PATH = $(PWD)/.build
 
-GO_VERSION := 1.1
-GOOS		   = $(subst Darwin,darwin,$(subst Linux,linux,$(OS)))
-GOARCH		 = $(subst x86_64,amd64,$(ARCH))
-GOPKG		   = go$(GO_VERSION).$(GOOS)-$(GOARCH).tar.gz
-GOROOT		 = $(BUILD_PATH)/root/go
-GOPATH		 = $(BUILD_PATH)/root/gopath
-GOCC		   = $(GOROOT)/bin/go
-TMPDIR		 = /tmp
-GOENV		   = TMPDIR=$(TMPDIR) GOROOT=$(GOROOT) GOPATH=$(GOPATH)
-GO	       = $(GOENV) $(GOCC)
-GOFMT		   = $(GOROOT)/bin/gofmt
+GO_VERSION	 := 1.1
+GOOS		  = $(subst Darwin,darwin,$(subst Linux,linux,$(OS)))
+GOARCH		  = $(subst x86_64,amd64,$(ARCH))
+GOPKG		 ?= go$(GO_VERSION).$(GOOS)-$(GOARCH).tar.gz
+GOURL		 ?= http://go.googlecode.com/files
+GOROOT		  = $(BUILD_PATH)/root/go
+GOPATH		  = $(BUILD_PATH)/root/gopath
+GOCC		  = $(GOROOT)/bin/go
+TMPDIR		  = /tmp
+GOENV		  = TMPDIR=$(TMPDIR) GOROOT=$(GOROOT) GOPATH=$(GOPATH)
+GO	          = $(GOENV) $(GOCC)
+GOFMT		  = $(GOROOT)/bin/gofmt
 
 LEVELDB_VERSION := 1.12.0
 PROTOCOL_BUFFERS_VERSION := 2.5.0