configure.in: size_t must be unsigned

* configure.in (RUBY_CHECK_SIGNEDNESS): macro to check signedness of a
  type.
* configure.in (size_t): must be unsigned.
  [ruby-core:57149] [Feature #8890]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42921 b2dd03c8-39d4-4d8f-98ff-823fe69b080e