fix go tarball download for OS X, use redirect from golang.org

Makefile

@@ -6,9 +6,22 @@ TARGET   := node_exporter
 OS         := $(subst Darwin,darwin,$(subst Linux,linux,$(shell uname)))
 ARCH       := $(subst x86_64,amd64,$(shell uname -m))
 
+# The release engineers apparently need to key their binary artifacts to the
+# Mac OS X release family.
+MAC_OS_X_VERSION ?= 10.8
+
 GOOS       ?= $(OS)
 GOARCH     ?= $(ARCH)
-GOPKG  := go1.4.1.$(OS)-$(ARCH).tar.gz
+
+ifeq ($(GOOS),darwin)
+RELEASE_SUFFIX ?= -osx$(MAC_OS_X_VERSION)
+else
+RELEASE_SUFFIX ?=
+endif
+	
+GO_VERSION ?= 1.4.1
+GOURL      ?= http://golang.org/dl
+GOPKG      ?= go$(GO_VERSION).$(GOOS)-$(GOARCH)$(RELEASE_SUFFIX).tar.gz
 GOROOT     := $(CURDIR)/.deps/go
 GOPATH     := $(CURDIR)/.deps/gopath
 GOCC       := $(GOROOT)/bin/go
@@ -24,7 +37,7 @@ default: $(BINARY)
 
 .deps/$(GOPKG):
 	mkdir -p .deps
-	curl -o .deps/$(GOPKG) https://storage.googleapis.com/golang/$(GOPKG)
+	curl -o .deps/$(GOPKG) -L $(GOURL)/$(GOPKG)
 
 $(GOCC): .deps/$(GOPKG)
 	tar -C .deps -xzf .deps/$(GOPKG)