better checking + warnings for doxygen

docs/CMakeLists.txt

@@ -5,8 +5,20 @@ file(DOWNLOAD https://raw.githubusercontent.com/jothepro/doxygen-awesome-css/v2.
      ${CMAKE_CURRENT_LIST_DIR}/doxygen-awesome-sidebar-only.css
      EXPECTED_HASH SHA256=998328b27193b7be007a431bc9be1a6f6855ff4d8fa722ecfdfed79a8931409f)
 
-add_custom_target(build_docs COMMAND doxygen ${CMAKE_CURRENT_LIST_DIR}/Doxyfile WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
-                  SOURCES ${CMAKE_CURRENT_LIST_DIR}/Doxyfile BYPRODUCTS ${CMAKE_BINARY_DIR}/html/index.html)
+execute_process(COMMAND doxygen --version RESULT_VARIABLE DOXYGEN_VERSION_RESULT
+                OUTPUT_VARIABLE DOXYGEN_VERSION_RAW_OUTPUT)
+if(NOT DOXYGEN_VERSION_RESULT EQUAL 0)
+  message(AUTHOR_WARNING "Unable to get the version of doxygen")
+else()
+  # Extracts the version from the output of the command run before
+  string(REGEX MATCH "^(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)" DOXYGEN_VERSION_OUTPUT
+               "${DOXYGEN_VERSION_RAW_OUTPUT}")
+  message(STATUS "doxygen version detected : ${DOXYGEN_VERSION_OUTPUT}")
+endif()
+
+add_custom_target(
+  build_docs COMMAND doxygen ${CMAKE_CURRENT_LIST_DIR}/Doxyfile WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+  SOURCES ${CMAKE_CURRENT_LIST_DIR}/Doxyfile BYPRODUCTS ${CMAKE_BINARY_DIR}/html/index.html)
 add_custom_command(
   TARGET build_docs POST_BUILD COMMAND ${CMAKE_COMMAND} -E cmake_echo_color --cyan
                                        "You can prview the documentation: ${CMAKE_BINARY_DIR}/html/index.html")