Why aren't BUILT_SOURCES removed by "make distclean"?

[Reuben Thomas]

I have to add $(BUILT_SOURCES) to CLEANFILES to avoid complaints about files 
that still exist after make distclean. Since the point of BUILT_SOURCES is 
to get sources built that aren't distributed, it would seem reasonable that 
they be cleaned up by make distclean. I'm using automake 1.10. Am I missing 
something? In the manual it mentions the heuristic rules for what gets 
removed by the various *clean targets, and since "make" builds these files, 
it seems reasonable that it should remove them.

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