message: Need to mkdir if we're building stanc3 locally too
diff: @@ -23,6 +23,7 @@ else
  endif
  else
  bin/stanc$(EXE) : $(shell find $(STANC3)/src/ -type f -name '*.ml*') $(STANC#)
  + @mkdir -p $(dir $@)
  cd $(STANC3) && echo "--- Rebuilding stanc ---\n" && dune build @install
  cp $(STANC3)/_build/default/src/stanc/stanc.exe $@
  endif
  