Yeah, FWIW, I've had to jump through a few hoops with older kernel releases on Kobo, too, even when using a GCC 4.9 TC.
Mostly simple/stupid fixes which are usually pretty obvious once you grok the error message

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Good to know that modules built w/ GCC 7 worked out fine, though

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And that Amazon finally stopped shipping kernel sources with missing non-trivial headers (because that was a thing at one point ^^).