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 <[email protected]>

Makefile

@@ -11,14 +11,14 @@
 #   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #   See the License for the specific language governing permissions and
 #   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}