Update ExtUtils-MakeMaker to CPAN version 6.68

  [DELTA]

6.68 Fri Jun 14 23:26:11 BST 2013
    No changes from 6.67_05

6.67_05 Thu Jun 13 21:52:46 BST 2013
    Doc fixes:
    * RT#86007 - Restore meaning for divorced sentence

6.67_04 Mon Jun 10 20:18:25 BST 2013
    Bug fixes
    * Address RT#85406, where specifying 'meta-spec' in
      META_[ADD|MERGE] would remove all prereqs (bingos)

6.67_03 Wed Jun  5 22:03:28 BST 2013
    Doc Fixes
    * Document how to specify meta-spec in META_MERGE (bingos)

6.67_02 Sun Jun  2 18:27:45 BST 2013
    Bug Fixes
    * Allow v-prefixed version strings once more (bingos)
    * Typos fixed (David Steinbrunner)
    * Resolve test failure with latest CPAN::Meta

6.67_01 Thu Apr 25 21:03:58 BST 2013
    Doc Fixes
    * Change references to makemaker.org in the docs
      (Reported as RT#83246 by dolmen)

    VOS Fixes
    * 'core' files are keep files (*.kp) on vos, adjust the
      'clean' target to account for this (Paul Green)

    Win32 Fixes
    * Increase dmake MAXLINELENGTH to 800000 (RT#77215) as
      per kmx's recommendations (kmx)

    Cygwin Fixes
    * Allow linking of Cygwin libraries (Reini Urban)