Disable CppAD optimization of conditional expressions (CSkipOp) by default
    - A bug was introduced when upgrading to recent cppad version.
      The bug affected examples using CppAD::CondExp (e.g. 'posfun').
      It is now fixed in CppAD and the fix has been merged into TMB.
    - For safety, we disable this optimization by default until the feature has been better tested.
    - To enable CSkipOp (for testing) add '-DTMB_ENABLE_CSKIPOP' to the pre-processor.
    - TODO: Add tmb_example that use conditional expressions (so that issues like
      this are caught by the automated tests in the future).