Export -DEIGEN_RUNTIME_NO_MALLOC flag for dependency

  function.hh defines EIGEN_RUNTIME_NO_MALLOC variable before including
  Eigen/Core. If a dependency includes Eigen/Core before function.hh, function
  set_is_malloc_allowed turns out to be undefined.
Merge pull request #50 from florent-lamiraux/master

Export -DEIGEN_RUNTIME_NO_MALLOC flag for dependency