configure.in: [Bug #8409]

* configure.in: check if prefix is same as the cwd, not only same
  path name.  also use AC_MSG_ERROR to bail out.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47152 b2dd03c8-39d4-4d8f-98ff-823fe69b080e