Allow setting git version at build time

I want to pass a flag to `go install`, e.g.

```
godep go install -tags heroku -ldflags "-X main.version $(git --git-dir=$(HEROKU_GIT_DIR) describe)"
```

I can't do this with only using environment variables, since the point here is to pass in compile time flags that potentially change at every compilation. Another example would be passing in the build time.

If there is another way to do this with the upstream buildpack, please let me know. If you think there is a better solution than a raw `Makefile`, I'm open to suggestions.

I have not added any documentation or test cases for this, as I wanted to get some feedback first.