prevent bogus complains about libevent version mismatch due to invalid VERSION...

prevent bogus complains about libevent version mismatch due to invalid VERSION define in libevent's config.h on windows

gevent/core.pyx

@@ -488,7 +488,7 @@ include "evdns.pxi"
 # XXX - make sure event queue is always initialized.
 init()
 
-if get_version() != get_header_version() and get_header_version() is not None:
+if get_version() != get_header_version() and get_header_version() is not None and get_version() != '1.3.99-trunk':
     import warnings
     msg = "libevent version mismatch: system version is %r but this gevent is compiled against %r" % (get_version(), get_header_version())
     warnings.warn(msg, UserWarning, stacklevel=2)