libev stub macros need to reflect void when disabled

Reported-by: Michael Harberler <mail17@mah.priv.at>
Signed-off-by: Andy Green <andy.green@linaro.org>
introduce LWS_FEATURE_ tests

Help user code be compatible with multiple LWS versions by
providing #ifdef -testable flags for API changes

Signed-off-by: Andy Green <andy.green@linaro.org>