the packaging apparently knows about the libgfortran issue:
# libgfortran doesn't handle FCFLAGS_FOR_TARGET, override FCFLAGS
FCFLAGS := $(shell $(dpkg_buildflags_target) --get FCFLAGS)
but the work-around is wrong.
the build log shows
checking whether the GNU Fortran compiler is working... no
configure: error: GNU Fortran is not working; please report a bug in
http://gcc.gnu.org/bugzilla, attaching /<<P
KGBUILDDIR>>/build/i686-w64-mingw32-posix/i686-w64-mingw32/libgfortran/config.log
but I couldn't find anything in the GCC bug tracker.
Now submitted https://gcc.gnu.org/PR127539
