mv include/ruby/{impl,internal}

Devs do not love "impl".

include/ruby/internal/constant_p.h

@@ -0,0 +1,37 @@
+#ifndef RBIMPL_CONSTANT_P_H                          /*-*-C++-*-vi:se ft=cpp:*/
+#define RBIMPL_CONSTANT_P_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      Defines #RBIMPL_CONSTANT_P.
+ *
+ * Note that __builtin_constant_p can be applicable inside of inline functions,
+ * according to GCC manual.  Clang lacks that feature, though.
+ *
+ * @see https://bugs.llvm.org/show_bug.cgi?id=4898
+ * @see https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
+ */
+#include "ruby/impl/has/builtin.h"
+
+#if RBIMPL_HAS_BUILTIN(__builtin_constant_p)
+# define RBIMPL_CONSTANT_P(expr) __builtin_constant_p(expr)
+#else
+# define RBIMPL_CONSTANT_P(expr) 0
+#endif
+
+#endif /* RBIMPL_CONSTANT_P_H */