Findmpi4py: add imported targets

CMake/Findmpi4py.cmake

+include(CMakeFindDependencyMacro)
+find_dependency(PythonInterp)
+execute_process(
+  COMMAND "${PYTHON_EXECUTABLE}" -c
+          "import mpi4py"
+  OUTPUT_VARIABLE _mpi4py_out
+  ERROR_VARIABLE  _mpi4py_out
+  RESULT_VARIABLE _mpi4py_res)
+if (_mpi4py_res)
+  set(mpi4py_FOUND 0)
+endif ()
+
+if (NOT mpi4py_INCLUDE_DIR OR NOT EXISTS "${mpi4py_INCLUDE_DIR}")
+  execute_process(
+    COMMAND "${PYTHON_EXECUTABLE}" -c
+            "import mpi4py; print(mpi4py.get_include())"
+    OUTPUT_VARIABLE _mpi4py_INCLUDE_DIR
+    RESULT_VARIABLE _mpi4py_include_dir_res
+    OUTPUT_STRIP_TRAILING_WHITESPACE)
+  if (_mpi4py_include_dir_res)
+    set(mpi4py_FOUND 0)
+  endif ()
+  set(mpi4py_INCLUDE_DIR "${_mpi4py_INCLUDE_DIR}"
+    CACHE INTERNAL "Path of the MPI4Py include directory")
+endif ()
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(mpi4py
+  REQUIRED_VARS mpi4py_INCLUDE_DIR)
+
+if (mpi4py_FOUND AND NOT TARGET mpi4py::mpi4py)
+  add_library(mpi4py::mpi4py INTERFACE IMPORTED)
+  set_target_properties(mpi4py::mpi4py
+    PROPERTIES
+      INTERFACE_INCLUDE_DIRECTORIES "${mpi4py_INCLUDE_DIR}")
+endif ()