Simplify MPI linkage handling (#591)

- find the C++ MPI component when configuring
- simplify MPI linkage and drop obsolete flag handling

Author: aobolensk

cmake/mpi.cmake

@@ -1,18 +1,8 @@
-find_package(MPI REQUIRED)
+find_package(MPI REQUIRED COMPONENTS CXX)
 if(NOT MPI_FOUND)
   message(FATAL_ERROR "MPI NOT FOUND")
 endif()
 
 function(ppc_link_mpi exec_func_lib)
-  if(MPI_COMPILE_FLAGS)
-    set_target_properties(${exec_func_lib} PROPERTIES COMPILE_FLAGS
-                                                      "${MPI_COMPILE_FLAGS}")
-  endif(MPI_COMPILE_FLAGS)
-
-  if(MPI_LINK_FLAGS)
-    set_target_properties(${exec_func_lib} PROPERTIES LINK_FLAGS
-                                                      "${MPI_LINK_FLAGS}")
-  endif(MPI_LINK_FLAGS)
-  target_include_directories(${exec_func_lib} PUBLIC ${MPI_INCLUDE_PATH})
-  target_link_libraries(${exec_func_lib} PUBLIC ${MPI_LIBRARIES})
+  target_link_libraries(${exec_func_lib} PUBLIC MPI::MPI_CXX)
 endfunction()