configure.in: clock_gettime

* configure.in (clock_gettime): should not overwrite cache variable
  with different condtion.  otherwise -lrt is not linked and the link
  fails, after reconfig.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42616 b2dd03c8-39d4-4d8f-98ff-823fe69b080e