libwebsockets

expose compiletime constants to setting from configure

This patch allows control of the main compiletime constants in libwebsockets
from the configure commandline.

README is updated with documentation on what's available, how to set them
and the defaults.

The constants are logged with "info" severity (not visible by default) at
context create time.

The zlib constant previously exposed like this is moved to private-libwebsockets.h
so it can be printed along with the rest.

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