app-cdr/gcombust needs, on amd64 (with which it works fine, btw) to have a libtoolize --force before configure (because the upstream tarball was made with an old libtool that doesnt recognize x86_64).
no real info here if the only bug is that the x86_64 HOST isnt being recognized, you dont fix that with libtoolize, you fix that with gnuconfig_update i updated the cascading profiles earlier so that all profiles now have FEATURES=autoconfig