configure.in: prereq 2.67

* configure.in: skip autoconf 2.64 and 2.66, 2.67 seems short-lived
  but stick on it for Debian Squeeze.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40034 b2dd03c8-39d4-4d8f-98ff-823fe69b080e