Fix compat to ruby < 3.0

Older rubies require the use of the rb_io_t structure for wait functions.
But older rb_io_wait_* functions are soft-deprecated, since they have to re-wrap the IO objects for the scheduler.
Moreover they don't fully support nonblocking behavior on Windows.

ext/extconf.rb

@@ -142,6 +142,7 @@ module PG
 have_func 'timegm'
 have_func 'rb_gc_adjust_memory_usage' # since ruby-2.4
 have_func 'rb_gc_mark_movable' # since ruby-2.7
+have_func 'rb_io_wait' # since ruby-3.0
 
 # unistd.h confilicts with ruby/win32.h when cross compiling for win32 and ruby 1.9.1
 have_header 'unistd.h'

ext/pg_connection.c

@@ -3101,6 +3101,17 @@ pgconn_get_last_result(VALUE self)
 	return rb_pgresult;
 }
 
+#if !defined(HAVE_RB_IO_WAIT)
+#define rb_io_wait(io, event, timeout) do { \
+	rb_io_t *fptr; \
+	GetOpenFile((io), fptr); \
+	if( (event) == RB_INT2NUM(RUBY_IO_READABLE) ) \
+		rb_io_wait_readable(fptr->fd); \
+	else \
+		rb_io_wait_writable(fptr->fd); \
+} while(false)
+#endif
+
 /*
  * call-seq:
  *    conn.discard_results()