ruby/ruby.h: fix for older gcc

* configure.in (__builtin_choose_expr_constant_p): in gcc 4.8 or
  earlier, __builtin_choose_expr() does not consider
  __builtin_constant_p(variable) a constant expression.

* include/ruby/ruby.h (RUBY_SAFE_LEVEL_CHECK): fix for older gcc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47867 b2dd03c8-39d4-4d8f-98ff-823fe69b080e