The 4.2 kernel has made flush_workqueue() GPL-only, which possibly affects every version of the nvidia-drivers package and causes the following compile error:
FATAL: modpost: GPL-incompatible module nvidia.ko uses GPL-only symbol 'flush_workqueue'
A patch has already been applied for 4.3 according to this discussion: https://lkml.org/lkml/2015/8/4/802
Here is the build.log http://pastebin.com/hwexnZAP
Same problem here on ~amd64.
gentoo-sources-4.2.0-r1 contains this patch. Please wait a few hours (as of this writing) for the genpatches and ebuild to hit your mirrors.
Latest patch confirmed fixed, built just fine for me.
Problem solved for me too with gentoo-sources-4.2.0-r1.