CPP: Fix CMakeLists to require Boost datetime needed by Boost thread.
CPP: Fix CMakeLists to require Boost datetime needed by Boost thread.

This also makes the library link with the Boost static libs on Windows.

Review URL: http://codereview.appspot.com/5430053
oneminot/libphonenumber,oneminot/libphonenumber,oneminot/libphonenumber,oneminot/libphonenumber