Makefile: support passing build tags via environment

build tags can be passed via GO_BUILDTAGS environment variable.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

Author: metux

Makefile

@@ -13,12 +13,13 @@
 #   limitations under the License.
 
 PACKAGES=$(shell go list ./... | grep -v /vendor/)
+GO_TAGS=$(if $(GO_BUILDTAGS),-tags "$(strip $(GO_BUILDTAGS))",)
 
 all: cgutil
-	go build -v
+	go build -v $(GO_TAGS)
 
 cgutil:
-	cd cmd/cgctl && go build -v
+	cd cmd/cgctl && go build -v $(GO_TAGS)
 
 proto:
 	protobuild --quiet ${PACKAGES}