README/Makefile: Mention & auto-install gox

README.md

@@ -39,7 +39,7 @@ Next, using [Git](https://git-scm.com/), clone this repository into `$GOPATH/src
 $ make
 ```
 
-To compile a development version of Terraform and the built-in plugins, run `make dev`. This will put Terraform binaries in the `bin` and `$GOPATH/bin` folders:
+To compile a development version of Terraform and the built-in plugins, run `make dev`. This will build everything using [gox](https://github.com/mitchellh/gox) and put Terraform binaries in the `bin` and `$GOPATH/bin` folders:
 
 ```sh
 $ make dev

scripts/build.sh

@@ -30,6 +30,11 @@ if [ "${TF_DEV}x" != "x" ]; then
     XC_ARCH=$(go env GOARCH)
 fi
 
+if ! which gox > /dev/null; then
+    echo "==> Installing gox..."
+    go get -u github.com/mitchellh/gox
+fi
+
 # Build!
 echo "==> Building..."
 gox \