configure.ac poll: Use an AC_MESG_WARN if !HAVE_POLL on non-Windows platforms

configure.ac

@@ -811,7 +811,12 @@ AC_CHECK_TYPE(ssize_t,, [
 ])
 
 dnl Check for poll
-AC_SEARCH_LIBS(poll, [poll], [AC_DEFINE(HAVE_POLL, 1, [Define to 1 if the OS is usabl... err, has poll().])])
+AC_SEARCH_LIBS(poll, [poll], [AC_DEFINE(HAVE_POLL, 1, [Define to 1 if the OS is usabl... err, has poll().])], [
+if test ${SYS} != "mingw32"
+then
+    AC_MSG_WARN([Your platform does not support poll(). VLC has it's own poll() implementation, but it is only intended to be used on Windows. VLC might crash or be insecure when you see this message. Either switch to an OS with a proper poll() implementation, or implement one for your OS in VLC])
+fi
+])
 
 dnl Check for dirent
 need_dirent=false