Subzero: Improve the "make check-presubmit" target.

Subzero: Improve the "make check-presubmit" target.

1. Include a test build (but no link) using g++, to identify errors and warnings before they hit the Windows bots.

2. Move "git diff --quiet" to the end so that the presubmit tests can easily be run before committing locally, if desired.

3. Add "make presubmit" as an alias for "make check-presubmit".

4. Document the individual steps.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1453713002 .