unix: Allow to override compiler warning options without touching the rest.

Some people want to enable even more warnings. Let them do it without putting
burden on everyone. Some people vice versa think that current settings should
be relaxed. In this regard, -Werror is the most problematic, it disallows to
use #warning directive, and disallows to pass configuration settings on make
command lines. Again, until decided how to deal with these globally, allow to
work around these problems locally.