glibmm went C++11-only in latest versions (which is what Debian Sid packages), so anything using glibmm must build with C++11 enabled where possible. hfst-ospell uses it.
Something like https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html can help with enabling C++11.
Fixed in r4457 + r4458