mkmf.rb: $extmk for symlink

* lib/mkmf.rb ($extmk): traverse parent directories for the case
  srcdir is a symlink.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38889 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-01-21

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38890 b2dd03c8-39d4-4d8f-98ff-823fe69b080e