url: don't compile detect_proxy if HTTP support is disabled

lib/url.c

@@ -4893,6 +4893,7 @@ static bool check_noproxy(const char *name, const char *no_proxy)
   return FALSE;
 }
 
+#ifndef CURL_DISABLE_HTTP
 /****************************************************************
 * Detect what (if any) proxy to use. Remember that this selects a host
 * name and is not limited to HTTP proxies only.
@@ -4962,6 +4963,7 @@ static char *detect_proxy(struct connectdata *conn)
 
   return proxy;
 }
+#endif /* CURL_DISABLE_HTTP */
 
 /*
  * If this is supposed to use a proxy, we need to figure out the proxy