enc: fundamental encindex

* enc/{ascii,us_ascii,utf_8}.c: set encoding indexes of
  fundamental built-in encodings so that usable as well as
  allocated rb_encoding before rb_enc_init().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51862 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

enc/utf_8.c

@@ -28,6 +28,10 @@
  */
 
 #include "regenc.h"
+#include "encindex.h"
+#ifndef ENCINDEX_UTF_8
+#define ENCINDEX_UTF_8 0
+#endif
 
 #define USE_INVALID_CODE_SCHEME
 
@@ -424,7 +428,7 @@ OnigEncodingDefine(utf_8, UTF_8) = {
   get_ctype_code_range,
   left_adjust_char_head,
   onigenc_always_true_is_allowed_reverse_match,
-  0,
+  ENCINDEX_UTF_8,
   ONIGENC_FLAG_UNICODE,
 };
 ENC_ALIAS("CP65001", "UTF-8")