Conditional append

[Shaun Jackman]

The automake files of my project may or may not define CLEANFILES,
they then include an automake snippet which conditionally appends a
file to CLEANFILES. I can't use either = or += because I get either

flash-image.am:4: CLEANFILES must be set with `=' before using `+='
    or
flash-image.am:4: CLEANFILES was already defined in condition TRUE,
which includes condition FLASH_IMAGE ...

How do I append to CLEANFILES in an included automake snippet?

Please cc me in your reply. Thanks,
Shaun