octave: fix linux install

we pass and assume a given MACHINE_GNU_PLATFORM, but this package did not
use this value due to an error.
the guess it had for linux (x86_64-unknown-linux-gnu) is different, and
this seems to trample over our own tools, see:
https://mail-index.netbsd.org/pkgsrc-users/2014/03/26/msg019464.html

add this patch to the original m4 script too, not just the resulting configure
file, so it won't be accidentially forgotten (and can be upstreamed).

patch originally by Iain Morgan
reminded by Dr. Thomas Orgis

(maya)