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

@@ -33,7 +33,8 @@ 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)-$(GOARCH).tar.gz
+GOPKG		 ?= go$(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