* configure.in: Don't link librt if clock_gettime is available in
  the main C library.
  glibc 2.17 moves clock_* from librt to the main C library.
  http://sourceware.org/ml/libc-announce/2012/msg00001.html



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40631 b2dd03c8-39d4-4d8f-98ff-823fe69b080e