glibmm failing to build

[Darrell Blake]

I'm trying to build mxe on a fresh Ubuntu LTS 20.04 by following the
tutorial but it's failing to build glibmm with the following error:

In file included from ../glibmm/containerhandle_shared.h:25,
                 from ../glibmm/arrayhandle.h:23,
                 from ../glibmm.h:91,
                 from balancedtree.cc:4:
../glibmm/variant.h:469:55: error: ISO C++17 does not allow dynamic
exception specifications
  469 |    static V_CastTo cast_dynamic(const VariantBase& v)
throw(std::bad_cast);
      |                                                       ^~~~~
../glibmm/variant.h:498:1: error: ISO C++17 does not allow dynamic
exception specifications
  498 | throw(std::bad_cast)
      | ^~~~~
../glibmm/variant.h:747:1: error: ISO C++17 does not allow dynamic
exception specifications
  747 | throw(std::bad_cast);
      | ^~~~~
../glibmm/variant.h:935:1: error: ISO C++17 does not allow dynamic
exception specifications
  935 | throw(std::bad_cast);
      | ^~~~~
../glibmm/variant.h:992:1: error: ISO C++17 does not allow dynamic
exception specifications
  992 | throw(std::bad_cast);
      | ^~~~~
make[3]: *** [Makefile:766: balancedtree.lo] Error 1
make[3]: Leaving directory
'/opt/mxe/tmp-glibmm-i686-w64-mingw32.static/glibmm-2.42.0/glib/glibmm'
make[2]: *** [Makefile:775: install-recursive] Error 1
make[2]: Leaving directory
'/opt/mxe/tmp-glibmm-i686-w64-mingw32.static/glibmm-2.42.0'
make[1]: *** [Makefile:881: build-only-glibmm_i686-w64-mingw32.static] Error 2
make[1]: Leaving directory '/opt/mxe'

Any ideas?