* process.c (rb_clock_gettime): add CLOCK_MONOTONIC support on OS X.
  http://developer.apple.com/library/mac/qa/qa1398/_index.html
  [Feature #8658]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42573 b2dd03c8-39d4-4d8f-98ff-823fe69b080e