cmake: probe gcc flags capability
cmake: probe gcc flags capability

old gcc doesn't have some of the extended warning flags we want to apply...
use cmake to confirm the gcc has them before trying to use them.
sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets,sartura/libwebsockets