The only relevant change in the environment I see is gcc-7.
According to https://tracker.debian.org/pkg/gcc-7:
"[2017-08-08] Accepted gcc-7 7.1.0-13 (source) into unstable"
openorienteering-mapper 0.6.71 FTBFS on unstable amd64 since 2017-08-08
(Don't know what really fails inside the test, but it loads an external
file, the path being determined using the __FILE__ macro. Hope the
compiler didn't break this.)