Add ENV.refurbish_args helper 

Rationale: our arg refurbishment is normally only turned on when called via the `make` wrapper, for compatibility reasons. However, there are numberous places we'd like this to be turned on elsewhere, like software that builds via `python setup.py` where bad flags from the system python can be pulled in.

This helper appends 'O' to CCCFG, which enables refurbishment for all calls of the compiler shims.

While the helper isn't much shorter than the line it replaces, it does make the intent clearer.