Use $GOOS

Makefile

@@ -2,19 +2,19 @@ DEPS = $(wildcard */*.go)
 VERSION = $(shell git describe --always --dirty)
 COMMIT_SHA1 = $(shell git rev-parse HEAD)
 BUILD_DATE = $(shell date +%Y-%m-%d)
-OS = linux
+GOOS = linux
 ARCH = amd64
 
 all: vendor lint vet prometheus-puppetdb-exporter
 
 prometheus-puppetdb-exporter: main.go $(DEPS)
-	GO111MODULE=on CGO_ENABLED=0 GOOS=linux \
+	GO111MODULE=on CGO_ENABLED=0 GOOS=$(GOOS) \
 	  go build -a \
 		  -ldflags="-X main.version=$(VERSION) -X main.commitSha1=$(COMMIT_SHA1) -X main.buildDate=$(BUILD_DATE)" \
 	    -installsuffix cgo -o $@ $<
 	strip $@
 
-release: prometheus-puppetdb-exporter-$(VERSION).$(OS)-$(ARCH).tar.gz
+release: prometheus-puppetdb-exporter-$(VERSION).$(GOOS)-$(ARCH).tar.gz
 
 %.tar.gz: prometheus-puppetdb-exporter LICENSE
 	tar cvzf $@ --transform 's,^,$*/,' $^