genkernel-3.1.0l+ breaks the modules_install phase when building a 2.4-series kernel. Basically it forces -j0 into MAKEOPTS, which is a bad thing(tm). It should probably do -j1.
Created attachment 52378 [details, diff] Fix/diff against genkernel-3.1.1
Oops, the yet unseen 3.1.1a fixes it. Closing.