Convert build script to a Makefile. (#24)

* Convert build script to a Makefile.

* Oops I forgot to delete build.sh

Makefile

@@ -0,0 +1,35 @@
+.PHONY: build build-linux build-mac build-windows release clean
+
+ROOT := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
+
+build: ## Build Slack Advanced Exporter for the current platform and architecture
+	go build .
+
+build-linux: ## Build Slack Advanced Exporter for Linux
+	@mkdir -p ${ROOT}build
+	cd ${ROOT}build && GOOS=linux GOARCH=amd64 go build ..
+
+build-mac: ## Build Slack Advanced Exporter for Mac
+	@mkdir -p ${ROOT}build
+	cd ${ROOT}build && GOOS=darwin GOARCH=amd64 go build ..
+
+build-windows: ## Build Slack Advanced Exporter for Windows
+	@mkdir -p ${ROOT}build
+	cd ${ROOT}build && GOOS=windows GOARCH=amd64 go build ..
+
+release: clean ## Build and package the release artefacts
+	@mkdir -p ${ROOT}build
+	@mkdir -p ${ROOT}release
+	cd ${ROOT}build && GOOS=linux GOARCH=amd64 go build ..
+	cd ${ROOT}build && tar -czf ../release/slack-advanced-exporter.linux-amd64.tar.gz slack-advanced-exporter
+	cd ${ROOT}build && GOOS=darwin GOARCH=amd64 go build ..
+	cd ${ROOT}build && tar -czf ../release/slack-advanced-exporter.darwin-amd64.tar.gz slack-advanced-exporter
+	cd ${ROOT}build && GOOS=windows GOARCH=amd64 go build ..
+	cd ${ROOT}build && zip -q ../release/slack-advanced-exporter.windows-amd64.zip slack-advanced-exporter.exe
+	cd ${ROOT}release && sha256sum ./slack-advanced-exporter.*
+
+clean: ## Remove all build and release artefacts
+	rm -rf build
+	rm -rf release
+	rm -rf slack-advanced-exporter
+	rm -rf slack-advanced-exporter.exe