Add option to disable -Werror for headers as well

Sometimes it is useful to build old versions of Qt, e.g for bisecting regressions.

The only issue when doing this is usually that new warnings have appeared since the last time the Qt version was updated (such as deprecation warnings). For some of these, we can use -no-warnings-are-error to force a build, but this doesn't work for public headers, which have an unconditional -Werror in the "clean" check.

For making debugging easier, it would be useful to have a "-no-warnings-are-errors-and-i-really-mean-it" option which also disables -Werror on the public headers.