Using a fresh installation, I installed genkernel and gentoo-sources. Upon finishing up with menuconfig, compilation failed of the kernel. The error stated `/bin/sh: bc: command not found`. I ran `which bc` which stated there was no bc command found on my system. I also did `emerge --search bc`, which showed there also was no sys-devel/bc installed on the system. The workaround for this is pretty simple. I needed to install sys-devel/bc. I however believe that this should be a dependency of genkernel since it's a direct linux kernel requirement. In particular, some files such as https://github.com/torvalds/linux/blob/master/kernel/time/timeconst.bc require it. The stage3 I installed was 20160204 and the version of genkernel I am using is 3.4.52.4. This seems to be an issue effecting all versions within the portage tree.
Genkernel does not need bc, but gentoo-sources does - it needs for kernel buildsystem. All kernel sources ebuilds depends on sys-devel/bc