* configure.in: Check __int128.

* include/ruby/defines.h (BDIGIT_DBL): Use uint128_t if it is available.
  (BDIGIT): Use uint64_t if uint128_t is available.
  (SIZEOF_BDIGITS): Defined for above case.
  (BDIGIT_DBL_SIGNED): Ditto.
  (PRI_BDIGIT_PREFIX): Ditto.

* include/ruby/ruby.h (PRI_64_PREFIX): Defined.

* bignum.c (rb_big_pow): Don't use BITSPERDIG for the condition which
  rb_big_pow returns Float or Bignum.

[ruby-dev:47413] [Feature #8509]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41379 b2dd03c8-39d4-4d8f-98ff-823fe69b080e