Access to compiler `stdout`

Access is needed to compiler `stdout` so that warnings may be inspected. A hook allowing post compile processes to crawl this output would allow existing warning checking scripts to be used.

Start by tracing how fab invokes compilers and handles their output. Define the hook's input and timing so post-compile processes can inspect compiler stdout and reuse warning-checking scripts; done means compiler output is accessible without breaking existing builds.

Written by the indexing model from the issue text.