Incompatible function types

[Dan Eble]

g++ 8 is spamming my build log with messages like this:

    /root/lilypond-src/lily/include/smobs.hh:228:37:
    warning: cast between incompatible function types
    from 'scm_unused_struct* (*)(SCM, SCM, SCM, SCM)’
    {aka 'scm_unused_struct* (*)(scm_unused_struct*,
    scm_unused_struct*, scm_unused_struct*, scm_unused_struct*)’}
    to 'scm_t_subr' {aka 'scm_unused_struct* (*)()’}
    [-Wcast-function-type]
                         (scm_t_subr)smob_trampoline<PMF>,
                                     ^~~~~~~~~~~~~~~~~~~~

The issue seems to be the differing number of parameters.
https://stackoverflow.com/q/559581
https://stackoverflow.com/q/188839

Do you see anything worth doing about it at this time?  I could possibly bring 
myself to ignore it, if you would recommend that.

Regards,
— 
Dan