[doxygen] Add auto-deletion of the html folder, so that doxygen always builds a new documentation

cmake/modules/DumuxDoxygen.cmake

@@ -4,8 +4,11 @@
 MACRO (add_dumux_doxygen_target)
   if(DOXYGEN_FOUND)
     add_doxygen_target()
+    add_custom_target(doxygen_${ProjectName}_prebuild
+                      COMMAND rm -rf ${CMAKE_BINARY_DIR}/doc/doxygen/html)
+    add_dependencies(doxygen_${ProjectName} doxygen_${ProjectName}_prebuild)
     add_custom_command(TARGET doxygen_${ProjectName}
                        POST_BUILD
                        COMMAND ${CMAKE_SOURCE_DIR}/doc/doxygen/sanitizelinks.sh)
   endif()
-ENDMACRO (add_dumux_doxygen_target)
+ENDMACRO ()