The manual currently says: "You should never explicitly mention the intermediate (C or C++) file in any `SOURCES' variable; only list the source file."
I don't know the code here, and this probably wasn't the question, but I think the manual's statement about "any `SOURCES' variables" was simply not meant to apply to BUILT_SOURCES (probably didn't think about it), but rather to the "normal" something_SOURCES variables. So my gut reaction is to add "(except @code{BUILT_SOURCES}, see below)". Later, it talks about adding the header file to "BUIILT_SOURCES". This seems at odds with what Mike said and I found empirically. I suggest that we could just say to add both files, although if you want to try to understand what's actually going on, more power to you :). Best, Karl