Use Pattern Rules in Makefile (#552)

Convert the release/ section of the Makefile to using pattern rules.

This facilitates building for other platforms.

Makefile

@@ -46,15 +46,13 @@ bench:
 	$(info INFO: Starting build $@)
 	go test -bench=.
 
-release/goss-linux-386: $(GO_FILES)
-	$(info INFO: Starting build $@)
-	CGO_ENABLED=0 GOOS=linux GOARCH=386 go build -ldflags "-X main.version=$(TRAVIS_TAG) -s -w" -o release/$(cmd)-linux-386 $(exe)
-release/goss-linux-amd64: $(GO_FILES)
-	$(info INFO: Starting build $@)
-	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -ldflags "-X main.version=$(TRAVIS_TAG) -s -w" -o release/$(cmd)-linux-amd64 $(exe)
-release/goss-linux-arm: $(GO_FILES)
-	$(info INFO: Starting build $@)
-	CGO_ENABLED=0 GOOS=linux GOARCH=arm go build -ldflags "-X main.version=$(TRAVIS_TAG) -s -w" -o release/$(cmd)-linux-arm $(exe)
+
+
+# Pattern rule for platform builds.
+# `subst` substitutes space for -, thus making an array
+# firstword, and word select indexes from said array.
+release/goss-%: $(GO_FILES)
+	CGO_ENABLED=0 GOOS=$(firstword $(subst -, ,$*)) GOARCH=$(word 2, $(subst -, ,$*)) go build -ldflags "-X main.version=$(TRAVIS_TAG) -s -w" -o $@ $(exe)
 
 release:
 	$(MAKE) clean