 2. Use the Stan compiler to generate C++ code, foo/bar.hpp.
    3. Compile the C++ code using cc . to generate foo/bar

  Additional make options:
    STANCFLAGS: defaults to &quot;&quot;. These are extra options passed to bin/stanc
      when generating C++ code. If you want to allow undefined functions in the
      Stan program, either add this to make/local or the command line:
          STANCFLAGS = --allow_undefined
    USER_HEADER: when STANCFLAGS has --allow_undefined, this is the name of the
      