From https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=853483, "libbson: ftbfs with GCC-7"

The package fails to build in a test rebuild on at least amd64 with
gcc-7/g++-7, but succeeds to build with gcc-6/g++-6. The
severity of this report may be raised before the buster release.
There is no need to fix this issue in time for the stretch release.

In the Debian build log, the last test that succeeds is "/endian/swap64", so the test that fails must be the next one, "/bson/iso8601/utc".

I rewrote the ISO8601 parser for CDRIVER-1962, but it's unknown whether the test would have passed with GCC 7 before my rewrite or not.

GCC 7 is unreleased. I think we just need to fix this before Debian Buster with GCC 7 arrives, or other GCC 7 distributions. It's possible it's a new GCC bug that will be resolved without us.