[cmake] Fix add_custom_commands to be compliant with cmake 3.31

doc/CMakeLists.txt

@@ -10,7 +10,7 @@ extract_flags(${PROJECT_NAME}_c BUILD_INTERFACE)
 separate_arguments(${PROJECT_NAME}_c_CXXFLAGS)
 macro(generate_docs header_file)
   add_custom_command(
-    TARGET ${PROJECT_NAME}_docs_cpp2rst
+    TARGET ${PROJECT_NAME}_docs_cpp2rst POST_BUILD
     COMMAND rm -rf ${CMAKE_CURRENT_SOURCE_DIR}/cpp2rst_generated
     COMMAND
       PYTHONPATH=${CPP2PY_BINARY_DIR}:$ENV{PYTHONPATH}
@@ -38,7 +38,7 @@ foreach(example ${ExampleList})
   add_executable(${PROJECT_NAME}_doc_${f} EXCLUDE_FROM_ALL ${example})
   set_property(TARGET ${PROJECT_NAME}_doc_${f} PROPERTY RUNTIME_OUTPUT_DIRECTORY ${d})
   target_link_libraries(${PROJECT_NAME}_doc_${f} triqs)
-  add_custom_command(TARGET ${PROJECT_NAME}_doc_${f}
+  add_custom_command(TARGET ${PROJECT_NAME}_doc_${f} POST_BUILD
    COMMAND ${PROJECT_NAME}_doc_${f} > ${CMAKE_CURRENT_SOURCE_DIR}/${d}/${f}.output 2>/dev/null
    WORKING_DIRECTORY ${d}
   )
@@ -55,7 +55,7 @@ endif()
 # Sphinx has internal caching, always run it
 add_custom_target(${PROJECT_NAME}_docs_sphinx ALL)
 add_custom_command(
-  TARGET ${PROJECT_NAME}_docs_sphinx
+  TARGET ${PROJECT_NAME}_docs_sphinx POST_BUILD
   COMMAND PYTHONPATH=${PROJECT_BINARY_DIR}/python:$ENV{PYTHONPATH} ${SPHINXBUILD_EXECUTABLE} -j auto -c . -b html ${CMAKE_CURRENT_SOURCE_DIR} html
 )