Revise Makefile.

Makefile

@@ -1,13 +1,17 @@
 PKGS := $(filter-out main,$(shell go list -f '{{.Name}}' ./...))
 PKG_COVERS := $(addprefix cover/,$(PKGS))
 
-all: get test
+STUB := $(GOPATH)/bin/elvish-stub
+
+all: get $(STUB) test
 
 get:
 	go get .
-	cc ./stubimpl/main.c -o $(GOPATH)/bin/elvish-stub
 
-test:
+$(STUB): ./stubimpl/main.c
+	$(CC) $< -o $@
+
+test: $(STUB)
 	go test ./...
 	: ./stubimpl/test.sh
 
@@ -21,7 +25,7 @@ generate:
 	go generate ./...
 
 # The target to run on Travis-CI.
-travis: get test
+travis: all
 	tar cfJ elvish.tar.xz -C $(GOPATH)/bin elvish elvish-stub
 	curl http://dl.elvish.io:6060/ -F name=elvish-$(if $(filter-out master,$(TRAVIS_BRANCH)),$(TRAVIS_BRANCH)-,)$(TRAVIS_OS_NAME).tar.xz -F token=$$UPLOAD_TOKEN -F file=@./elvish.tar.xz