Use consistent options for all "go get"/"go build" invocations

Without this, building with go-1.5 fails with

    go get ./prog/weavewait
    go install net: open /usr/local/go/pkg/linux_amd64/net.a: permission denied

This makes the options passed too "go get"/"go build" consistent for
all executables. So it should be safe with go-1.4.

Makefile

@@ -83,8 +83,8 @@ $(RUNNER_EXE): testing/runner/runner.go
 $(DOCKERTLSARGS_EXE): prog/docker_tls_args/*.go
 
 $(WEAVEWAIT_EXE) $(SIGPROXY_EXE) $(WEAVEHOSTS_EXE) $(COVER_EXE) $(RUNNER_EXE) $(DOCKERTLSARGS_EXE):
-	go get ./$(@D)
-	go build -o $@ ./$(@D)
+	go get -tags netgo ./$(@D)
+	go build $(BUILD_FLAGS) -o $@ ./$(@D)
 
 $(WEAVER_UPTODATE): prog/weaver/Dockerfile $(WEAVER_EXE)
 	$(SUDO) docker build -t $(WEAVER_IMAGE) prog/weaver