bug: run cmake again after failed attempt

pybindmagic/__init__.py

@@ -99,8 +99,9 @@ def cpp(line, cell, *what):
     if cmake_path is None:
         ensure_tmp_folder()
     else:
-        run_cmake = not ensure_build_dir(cmake_path, clear=rebuild)
+        ensure_build_dir(cmake_path, clear=rebuild)
         p = os.path.abspath(os.path.join(cmake_path,"build"))
+        run_cmake = not os.path.exists(os.path.join(p, "Makefile"))
         if p not in sys.path:
             sys.path.append(p)
         ensure_pybind(cmake_path)