Distinguish automake substitutions from config.status

Distinguish automake substitutions from config.status
substitutions.
* automake.in (&add_depend2): Transform AMDEP.
(&handle_clean): Transform MCFILES and MFILES.
(&file_contents): Transform MAINTAINER_MODE.
(&transform, &am_install_var): Use `%', not `@'.
Adjust all the *.am files.
* clean.am: Use ?MFILES? instead of ad hoc MAINTAINERCLEAN.
* depend2.am: Display the double dependency on both ?AMDEP? and
@AMDEP@.