Build fails with octave 10

[doronbehar]

Hello, on NixOS we encountered the following failure when trying to build ltfat with octave 10.1.0:

https://gist.github.com/doronbehar/76dafbe2fdfd014866f80942d5e00d03

[dasergatskov]

See https://github.com/ltfat/ltfat/issues/189

[allthatsounds]

Thanks @dasergatskov ! And sorry @doronbehar for being so unresponsive. I agree that #189 is at least part of the solution, unfortunately, we do not have the resources to fix this right now (more precisely, to ensure that after fixing, everything works as expected, to do a new release, and to manage the questions potentially arising therefrom). We are working on a solution for this.

[StefanBruens]

For octave builds it is sufficient to replace 'gnu++11' with 'gnu++17' in oct/Makefile_unix.