the patch calls system bc but that means it needs system bc to be installed https://gitweb.gentoo.org/repo/gentoo.git/tree/sys-devel/bc/files/bc-1.07.1-use-system-bc.patch
while true, there isn't really a way to express this since it's a self-circular loop when not cross-compiling. ebuilds have no way of expressing "only when cross-compiling" in dependencies.
What we tend to do is build one for CBUILD within the ebuild if it's not already installed.