Configure: changed default compiler from "gcc" to "cc".
    
This allows to automatically pick the preferred system compiler on
systems with multiple compilers.

Reviewed by:	mdounin, ru
Proxy: support for connection upgrade (101 Switching Protocols).

This allows to proxy WebSockets by using configuration like this:

    location /chat/ {
        proxy_pass http://backend;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
    }

Connection upgrade is allowed as long as it was requested by a client
via the Upgrade request header.
Proxy: fixed do_write handling in previous commit.

As rightfully complained by MSVC, do_write variable was used uninitialized.
Correct fix is to set it's initial value based on event happened.