linbox master does not build on cygwin

[mkoeppe]

/usr/bin/bash ../libtool  --tag=CXX   --mode=link g++ -std=gnu++11 -O2 -Wall -DNDEBUG -UDEBUG -DFFLAS_COMPILED -DFFPACK_COMPILED  -I/opt/sage-fflas-ffpack-3c2193e3d42d64d0be6bf6eba82e46c84540b49f/include   -O2 -g -march=native  -L/opt/sage-fflas-ffpack-3c2193e3d42d64d0be6bf6eba82e46c84540b49f/lib -Wl,-rpath,/opt/sage-fflas-ffpack-3c2193e3d42d64d0be6bf6eba82e46c84540b49f/lib  -o liblinbox.la -rpath /opt/sage-fflas-ffpack-3c2193e3d42d64d0be6bf6eba82e46c84540b49f/lib dummy.lo util/libutil.la randiter/libranditer.la algorithms/libalgorithms.la -lntl -lmpfr  -L/opt/sage-fflas-ffpack-3c2193e3d42d64d0be6bf6eba82e46c84540b49f/lib -liml -lflint  
libtool:   error: can't build x86_64-unknown-cygwin shared library unless -no-undefined is specified

(from https://github.com/linbox-team/fflas-ffpack/runs/2209621554?check_suite_focus=true)