Ok, the problem was that CFLAGS now contains -Werror=implicit-function-declaration by default, set by dpkg-buildflags in unstable.
The minimal fix is to drop such option, by adding this line to debian/rules: export DEB_CFLAGS_MAINT_STRIP = -Werror=implicit-function-declaration I'm Cc:ing Andrey Rakhmatullin, who was kind enough to look at this and try to diagnose it. Thanks.
