Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0
Thanks for the heads-up. Clearly my attempt at supporting a hand-built system is
more trouble than it's worth, since it breaks the build so often elsewhere, so
I've reverted the patch. I'm not sure how to proceed, other than suggesting
special flags for the hand-built approach.