configure.in: rb_cv_mcontext_t_ptr

* configure.in (rb_cv_mcontext_t_ptr): abstract whether mcontext_t
  is a pointer type.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48826 b2dd03c8-39d4-4d8f-98ff-823fe69b080e