mingw add win32helper to lib and fix extpoll

Based on work from Radu Sorici <soriciradu@gmail.com>

Signed-off-by: Andy Green <andy.green@linaro.org>
diff --git a/libwebsockets-api-doc.html b/libwebsockets-api-doc.html
index 98e1e66..afffe0d 100644
--- a/libwebsockets-api-doc.html
+++ b/libwebsockets-api-doc.html
@@ -863,6 +863,7 @@
 &nbsp; &nbsp; <i>const char *</i> <b>name</b>;<br>
 &nbsp; &nbsp; <i>int (*</i><b>callback</b>) <i>(struct libwebsocket_context *context,struct libwebsocket_extension *ext,struct libwebsocket *wsi,enum libwebsocket_extension_callback_reasons reason,void *user, void *in, size_t len)</i>;<br>
 &nbsp; &nbsp; <i>size_t</i> <b>per_session_data_size</b>;<br>
+&nbsp; &nbsp; <i>void *</i> <b>per_context_private_data</b>;<br>
 };<br>
 <h3>Members</h3>
 <dl>
@@ -874,5 +875,9 @@
 <dd>Libwebsockets will auto-malloc this much
 memory for the use of the extension, a pointer
 to it comes in the <tt><b>user</b></tt> callback parameter
+<dt><b>per_context_private_data</b>
+<dd>Optional storage for this externsion that
+is per-context, so it can track stuff across
+all sessions, etc, if it wants
 </dl>
 <hr>