[cmake][bugfix] Really purge Doxylocal from build system if it wasn't found

This fixes the incomplete MR !61.

cmake/modules/DuneDoxygen.cmake

@@ -54,7 +54,7 @@ MACRO (prepare_doxyfile)
       COMMENT "Creating Doxyfile.in"
       DEPENDS ${DOXYSTYLE_FILE} ${CMAKE_CURRENT_SOURCE_DIR}/Doxylocal)
   else()
-    set(make_doxyfile_command ${CMAKE_COMMAND} -D DOT_TRUE=${DOT_TRUE} -D DUNEWEB_TRUE=\# -D ProjectName=${ProjectName} -D DUNE_MOD_VERSION=${DUNE_MOD_VERSION} -D DOXYSTYLE=${DOXYSTYLE_FILE}  -D DOXYLOCAL=${CMAKE_CURRENT_SOURCE_DIR}/Doxylocal -D abs_top_srcdir=${CMAKE_SOURCE_DIR} -D top_srcdir=${CMAKE_SOURCE_DIR} -P ${SCRIPT_DIR}/CreateDoxyFile.cmake)
+    set(make_doxyfile_command ${CMAKE_COMMAND} -D DOT_TRUE=${DOT_TRUE} -D DUNEWEB_TRUE=\# -D ProjectName=${ProjectName} -D DUNE_MOD_VERSION=${DUNE_MOD_VERSION} -D DOXYSTYLE=${DOXYSTYLE_FILE} -D abs_top_srcdir=${CMAKE_SOURCE_DIR} -D top_srcdir=${CMAKE_SOURCE_DIR} -P ${SCRIPT_DIR}/CreateDoxyFile.cmake)
     add_custom_command (OUTPUT Doxyfile.in Doxyfile
       COMMAND ${make_doxyfile_command}
       COMMENT "Creating Doxyfile.in"