extension libraries.

* bignum.c (bignew_1, bigadd): K&R style argument actually can't be
  defined as char.

* missing/vsnprintf.c: ANSI compiler supports const keyword.

* ext/digest/sha2/extconf.rb: reject platforms which has inttypes.h
  but no 64bit integer.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@9089 b2dd03c8-39d4-4d8f-98ff-823fe69b080e