configure_me: conditional compilation

Add the ability to write e.g. `cfg = "target_os = \"linux\""` in [`configure_me`](https://github.com/Kixunil/configure_me) specification to make a flag/option only available when compiled with certain feature. Bonus point for adding `inform_cfg_off`, which would print "The flag `--foo` is unavailable because this application was compiled without support for this feature" instead of "unknown flag" message.

Start by locating the configure_me specification handling and how flags or options are made available during compilation. Implement the requested cfg-based conditional availability, then verify that unavailable flags are excluded; if the bonus is attempted, confirm inform_cfg_off produces the specified message instead of "unknown flag".

Written by the indexing model from the issue text.