Makefile: allow for gotip (#928)

This allows `make GO=gotip build` to work.

Author: dveeden

Makefile

@@ -1,30 +1,31 @@
 all: build
 
 GO111MODULE=on
+MYSQL_VERSION ?= 8.0
+GO ?= go
 
 build:
-	go build -o bin/go-mysqlbinlog cmd/go-mysqlbinlog/main.go
-	go build -o bin/go-mysqldump cmd/go-mysqldump/main.go
-	go build -o bin/go-canal cmd/go-canal/main.go
-	go build -o bin/go-binlogparser cmd/go-binlogparser/main.go
+	${GO} build -o bin/go-mysqlbinlog cmd/go-mysqlbinlog/main.go
+	${GO} build -o bin/go-mysqldump cmd/go-mysqldump/main.go
+	${GO} build -o bin/go-canal cmd/go-canal/main.go
+	${GO} build -o bin/go-binlogparser cmd/go-binlogparser/main.go
 
 test:
-	go test --race -timeout 2m ./...
+	${GO} test --race -timeout 2m ./...
 
-MYSQL_VERSION ?= 8.0
 test-local:
 	docker run --rm -d --network=host --name go-mysql-server \
 		-e MYSQL_ALLOW_EMPTY_PASSWORD=true \
 		-e MYSQL_DATABASE=test \
 		-v $${PWD}/docker/resources/replication.cnf:/etc/mysql/conf.d/replication.cnf \
 		mysql:$(MYSQL_VERSION)
 	docker/resources/waitfor.sh 127.0.0.1 3306 \
-		&& go test -race -v -timeout 2m ./...
+		&& ${GO} test -race -v -timeout 2m ./...
 	docker stop go-mysql-server
 
 fmt:
 	golangci-lint run --fix
 
 clean:
-	go clean -i ./...
+	${GO} clean -i ./...
 	@rm -rf ./bin