ext/nio4r/extconf.rb: check for port_event_t in port.h (fixes #178)

"port.h" is a pretty generic name, and apparently has caused nio4r to
fail to build in practice.

Before trying to use it, make sure it contains the types we expect (for
Solaris I/O completion ports)

ext/nio4r/extconf.rb

@@ -14,7 +14,7 @@
 
 $defs << "-DEV_USE_SELECT" if have_header("sys/select.h")
 
-$defs << "-DEV_USE_POLL" if have_header("poll.h")
+$defs << "-DEV_USE_POLL" if have_type("port_event_t", "poll.h")
 
 $defs << "-DEV_USE_EPOLL" if have_header("sys/epoll.h")