Add missing include

45e65f3 made
ruby/internal/intern/thread.h depend on rb_wait_for_single_fd, which is
declared in ruby/io.h. However, it cannot include ruby/io.h directly due
to circular dependency issues.  So add an ruby/internal/deprecated_io.h
file with rb_wait_for_single_fd and a couple similar functions, and
have both ruby/io.h and ruby/internal/intern/thread.h include it.

Fixes [Bug #18046]

include/ruby/internal/deprecated_io.h

@@ -0,0 +1,38 @@
+#ifndef RBIMPL_DEPRECATED_IO_H                           /*-*-C++-*-vi:se ft=cpp:*/
+#define RBIMPL_DEPRECATED_IO_H
+/**
+ * @file
+ * @author     Ruby developers <ruby-core@ruby-lang.org>
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ * @warning    Symbols   prefixed  with   either  `RBIMPL`   or  `rbimpl`   are
+ *             implementation details.   Don't take  them as canon.  They could
+ *             rapidly appear then vanish.  The name (path) of this header file
+ *             is also an  implementation detail.  Do not expect  it to persist
+ *             at the place it is now.  Developers are free to move it anywhere
+ *             anytime at will.
+ * @note       To  ruby-core:  remember  that   this  header  can  be  possibly
+ *             recursively included  from extension  libraries written  in C++.
+ *             Do not  expect for  instance `__VA_ARGS__` is  always available.
+ *             We assume C99  for ruby itself but we don't  assume languages of
+ *             extension libraries. They could be written in C++98.
+ * @brief      Deprecated functions related to IO, with separate file used to
+ *             avoid circular dependencies.
+ */
+
+RBIMPL_SYMBOL_EXPORT_BEGIN()
+
+//RBIMPL_ATTR_DEPRECATED(("use rb_io_maybe_wait_readable"))
+int rb_io_wait_readable(int fd);
+
+//RBIMPL_ATTR_DEPRECATED(("use rb_io_maybe_wait_writable"))
+int rb_io_wait_writable(int fd);
+
+//RBIMPL_ATTR_DEPRECATED(("use rb_io_wait"))
+int rb_wait_for_single_fd(int fd, int events, struct timeval *tv);
+
+RBIMPL_SYMBOL_EXPORT_END()
+
+#endif /* RBIMPL_DEPRECATED_IO_H */

include/ruby/internal/intern/thread.h

@@ -20,6 +20,7 @@
  *             extension libraries. They could be written in C++98.
  * @brief      Public APIs related to ::rb_cThread.
  */
+#include "ruby/internal/deprecated_io.h"
 #include "ruby/internal/config.h"
 #include "ruby/internal/cast.h"
 #include "ruby/internal/dllexport.h"

include/ruby/io.h

@@ -154,14 +154,7 @@ int rb_io_extract_encoding_option(VALUE opt, rb_encoding **enc_p, rb_encoding **
 void rb_io_extract_modeenc(VALUE *vmode_p, VALUE *vperm_p, VALUE opthash, int *oflags_p, int *fmode_p, rb_io_enc_t *convconfig_p);
 ssize_t rb_io_bufwrite(VALUE io, const void *buf, size_t size);
 
-//RBIMPL_ATTR_DEPRECATED(("use rb_io_maybe_wait_readable"))
-int rb_io_wait_readable(int fd);
-
-//RBIMPL_ATTR_DEPRECATED(("use rb_io_maybe_wait_writable"))
-int rb_io_wait_writable(int fd);
-
-//RBIMPL_ATTR_DEPRECATED(("use rb_io_wait"))
-int rb_wait_for_single_fd(int fd, int events, struct timeval *tv);
+#include "ruby/internal/deprecated_io.h"
 
 VALUE rb_io_wait(VALUE io, VALUE events, VALUE timeout);
 VALUE rb_io_maybe_wait(int error, VALUE io, VALUE events, VALUE timeout);