Revised handling of unknown warnings

[xavierleroy]

Currently, unrecognized warning options -Wsome-unknown-warning are fatal errors.

As suggested in #552, this PR makes sure that -Wsome-unknown-warning is a warning (by default), and that -Wno-some-unknown-warning is ignored. This is what GCC and Clang do.

This commit also introduces the unknown-warning-option warning, so that warnings about unknown warnings can be ignored (-Wno-unknown-warning-option) or turned into an error (-Werror=unknown-warning-option). This is what Clang does.

Closes: #552