split internal.h into files

One day, I could not resist the way it was written.  I finally started
to make the code clean.  This changeset is the beginning of a series of
housekeeping commits.  It is a simple refactoring; split internal.h into
files, so that we can divide and concur in the upcoming commits.  No
lines of codes are either added or removed, except the obvious file
headers/footers.  The generated binary is identical to the one before.

internal/compilers.h

@@ -0,0 +1,87 @@
+#ifndef INTERNAL_COMPILERS_H /* -*- C -*- */
+#define INTERNAL_COMPILERS_H
+/**
+ * @file
+ * @brief      Internal header absorbing C compipler differences.
+ * @author     \@shyouhei
+ * @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.
+ */
+
+#ifndef MAYBE_UNUSED
+# define MAYBE_UNUSED(x) x
+#endif
+
+#ifndef WARN_UNUSED_RESULT
+# define WARN_UNUSED_RESULT(x) x
+#endif
+
+#ifndef __has_feature
+# define __has_feature(x) 0
+#endif
+
+#ifndef __has_extension
+# define __has_extension __has_feature
+#endif
+
+#define RB_OBJ_BUILTIN_TYPE(obj) rb_obj_builtin_type(obj)
+#define OBJ_BUILTIN_TYPE(obj) RB_OBJ_BUILTIN_TYPE(obj)
+#ifdef __GNUC__
+#define rb_obj_builtin_type(obj) \
+__extension__({ \
+    VALUE arg_obj = (obj); \
+    RB_SPECIAL_CONST_P(arg_obj) ? -1 : \
+        RB_BUILTIN_TYPE(arg_obj); \
+    })
+#else
+static inline int
+rb_obj_builtin_type(VALUE obj)
+{
+    return RB_SPECIAL_CONST_P(obj) ? -1 :
+        RB_BUILTIN_TYPE(obj);
+}
+#endif
+
+/* A macro for defining a flexible array, like: VALUE ary[FLEX_ARY_LEN]; */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
+# define FLEX_ARY_LEN   /* VALUE ary[]; */
+#elif defined(__GNUC__) && !defined(__STRICT_ANSI__)
+# define FLEX_ARY_LEN 0 /* VALUE ary[0]; */
+#else
+# define FLEX_ARY_LEN 1 /* VALUE ary[1]; */
+#endif
+
+/*
+ * For declaring bitfields out of non-unsigned int types:
+ *   struct date {
+ *      BITFIELD(enum months, month, 4);
+ *      ...
+ *   };
+ */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
+# define BITFIELD(type, name, size) type name : size
+#else
+# define BITFIELD(type, name, size) unsigned int name : size
+#endif
+
+#if defined(USE_UNALIGNED_MEMBER_ACCESS) && USE_UNALIGNED_MEMBER_ACCESS && \
+    (defined(__clang__) || GCC_VERSION_SINCE(9, 0, 0))
+#include "warnings.h"
+# define UNALIGNED_MEMBER_ACCESS(expr) __extension__({ \
+    COMPILER_WARNING_PUSH; \
+    COMPILER_WARNING_IGNORED(-Waddress-of-packed-member); \
+    typeof(expr) unaligned_member_access_result = (expr); \
+    COMPILER_WARNING_POP; \
+    unaligned_member_access_result; \
+})
+#else
+# define UNALIGNED_MEMBER_ACCESS(expr) expr
+#endif
+#define UNALIGNED_MEMBER_PTR(ptr, mem) UNALIGNED_MEMBER_ACCESS(&(ptr)->mem)
+
+#undef RB_OBJ_WRITE
+#define RB_OBJ_WRITE(a, slot, b) UNALIGNED_MEMBER_ACCESS(rb_obj_write((VALUE)(a), (VALUE *)(slot), (VALUE)(b), __FILE__, __LINE__))
+
+#endif /* INTERNAL_COMPILERS_H */