do not pollute CI with useless warnings/errors

Documentation/doc/CMakeLists.txt

@@ -354,6 +354,10 @@ configure_file(${CGAL_DOC_RESOURCE_DIR}/BaseDoxyfile.in
 
 set(CGAL_DOC_DOXY_DEFAULT "${CGAL_DOC_DXY_DIR}/BaseDoxyfile")
 
+if (BE_QUIET)
+  file(APPEND ${CGAL_DOC_DOXY_DEFAULT} "WARN_LOGFILE=doxygen.log\n")
+endif()
+
 # pkglist_filter gets the path to the pkglist_filter of this source
 # directory.
 if(WIN32)

Installation/CMakeLists.txt

@@ -1052,7 +1052,7 @@ You must disable CGAL_ENABLE_CHECK_HEADERS.")
     execute_process(
       COMMAND
         "${CMAKE_COMMAND}" -DCGAL_BUILD_THREE_DOC=TRUE
-        -DDOXYGEN_EXECUTABLE=${DOXYGEN_EXECUTABLE}
+        -DDOXYGEN_EXECUTABLE=${DOXYGEN_EXECUTABLE} -DBE_QUIET=TRUE -DDOXYGEN_QUIET=TRUE
         "${CGAL_SOURCE_DIR}/Documentation/doc"
       WORKING_DIRECTORY "${DOC_DIR}")
     execute_process(