introduce without extensions

The new --without-extensions config flag completely removes all code
and data related to extensions from the build throughout the library
when given.

Signed-off-by: Andy Green <andy.green@linaro.org>
diff --git a/libwebsockets-api-doc.html b/libwebsockets-api-doc.html
index fcd92cd..d907e9b 100644
--- a/libwebsockets-api-doc.html
+++ b/libwebsockets-api-doc.html
@@ -245,7 +245,8 @@
 It's not const because we write the owning_server member
 <dt><b>extensions</b>
 <dd>NULL or array of libwebsocket_extension structs listing the
-extensions this context supports
+extensions this context supports.  If you configured with
+--without-extensions, you should give NULL here.
 <dt><b>ssl_cert_filepath</b>
 <dd>If libwebsockets was compiled to use ssl, and you want
 to listen using SSL, set to the filepath to fetch the