unionfs 1.3 is associated with recent kernels, but it doesn't compile under 64-bit (e.g., amd64) because of a bug. This bug is addressed with a patch provided by the developer. Details are at https://bugzilla.fsl.cs.sunysb.edu/show_bug.cgi?id=521. Can you please add to the unionfs ebuild?
*** This bug has been marked as a duplicate of 142517 ***