This upstream commit fixes the test failures on s390x: https://github.com/oSoMoN/fontforge/commit/fde85b13382595cb3ab889e38570b4944edad808 .
I cherry-picked it and applied it to 1:20190801~dfsg-5 (only the last hunk doesn't apply cleanly, it can be removed), and the build succeeded on amd64, arm64, armhf, ppc64el and s390x (against Ubuntu groovy-proposed).
