libstdc++: Allow net::io_context to compile without <poll.h> [PR 100180]

This adds dummy placeholders to net::io_context so that it can still be
compiled on targets without <poll.h>.

libstdc++-v3/ChangeLog:

	PR libstdc++/100180
	* include/experimental/io_context (io_context): Define
	dummy_pollfd type so that most member functions still compile
	without <poll.h> and struct pollfd.

libstdc++-v3/include/experimental/io_context

@@ -716,6 +716,7 @@ inline namespace v1
 
     struct __reactor
     {
+#ifdef _GLIBCXX_HAVE_POLL_H
       __reactor() : _M_fds(1)
       {
 	int __pipe[2];
@@ -739,6 +740,7 @@ inline namespace v1
 	::close(_M_fds.back().fd);
 	::close(_M_notify_wr);
       }
+#endif
 
       // write a notification byte to the pipe (ignoring errors)
       void _M_notify()
@@ -799,8 +801,12 @@ inline namespace v1
 	_M_notify();
       }
 
-# ifdef _GLIBCXX_HAVE_POLL_H
+#ifdef _GLIBCXX_HAVE_POLL_H
       using __fdvec = vector<::pollfd>;
+#else
+      struct dummy_pollfd { int fd = -1; short events = 0, revents = 0; };
+      using __fdvec = vector<dummy_pollfd>;
+#endif
 
       // Find first element p such that !(p.fd < __fd)
       // N.B. always returns a dereferencable iterator.
@@ -816,6 +822,7 @@ inline namespace v1
       __status
       wait(__fdvec& __fds, chrono::milliseconds __timeout)
       {
+#ifdef _GLIBCXX_HAVE_POLL_H
 	// XXX not thread-safe!
 	__fds = _M_fds;  // take snapshot to pass to poll()
 
@@ -845,10 +852,14 @@ inline namespace v1
 	__fds.erase(__part, __fds.end());
 
 	return _S_ok;
+#else
+	(void) __timeout;
+	__fds.clear();
+	return _S_error;
+#endif
       }
 
       __fdvec _M_fds;	// _M_fds.back() is the read end of the self-pipe
-#endif
       int _M_notify_wr;	// write end of the self-pipe
     };