Port seaf-server and httpserver to MinGW
Eliminate some warnings in Windows
[win] Use proper file system APIs.
Fix abug for pack source code
[server] Improve configure.ac

Add --enable-mysql config option.
[win] Fix a few bugs for server.

* evbuffer_write() doesn't work for file on Windows;
* bufferevent_new_socket() requires socket be non-blocking;
* Provide --disable-client config option on Windows;
* Remove --enable-httpserver config option, default when --enable-server is true.
Fix a typo
Fix a bug for make dist