(To the administrators: it would be nice to have a sticky po-box here, as I assume these problems will continue to rise.)
I tend to build my own kernel from some git head merges, to keep in touch with latest improvements of the linux kernel.
It turned out, that my gentoo box fails to compile the 295.40 driver for some time now.
The attached patch makes the gentoo package compile again and, as the gentoo package is just a semi-automatic make script for the original source-package, it's quite likely that the original source-package can be patched this way too:
@NVidia Developers: It would be nice to detect this kernel update, but I tend not to fiddle with configurators, so I leave it up to you to fix your codebase.