Makefile: Read GO_BUILD_FLAGS from env and prepend

Build flags read from env GO_BUILD_FLAGS are prepended to
flags in Makefile and sent to scripts/build.sh for building
executables (etcd, etcdctl & etcdutl).

Signed-off-by: Bhargav Ravuri <bhargav.ravuri@infracloud.io>

CONTRIBUTING.md

@@ -57,6 +57,11 @@ Setup environment:
   - For ubuntu and debian run `sudo apt-get install build-essentials`
 - Verify that everything is installed by running `make build`
 
+Note: `make build` runs with `-v`. Other build flags can be added through env `GO_BUILD_FLAGS`, **if required**. Eg.,
+```console
+GO_BUILD_FLAGS="-buildmode=pie" make build
+```
+
 [file an issue]: https://github.com/etcd-io/etcd/issues/new/choose
 
 ## Implement your change

Makefile

@@ -1,6 +1,6 @@
 .PHONY: build
 build:
-	GO_BUILD_FLAGS="-v" ./scripts/build.sh
+	GO_BUILD_FLAGS="${GO_BUILD_FLAGS} -v" ./scripts/build.sh
 	./bin/etcd --version
 	./bin/etcdctl version
 	./bin/etcdutl version