Virtualbox: building with kernel built with make O=

[mounty1]

Because I have multiple machines, to save space I emerge gentoo-sources just once onto a shared network drive, then use make O= to build the distinct versions of the kernel, as required. This spoofs xf86-virtualbox-video though, because it starts rummaging in /usr/src/linux for certains headers which are actually in locations part-specified by the argument to O. For example, one build is make O=dickless (ho yes, very amusing) and headers are generated in /usr/src/linux/dickless/include instead of /usr/src/linux/include.

Is there a way to tell the build about this ? After looking in the scripts, it seemed that setting KERN_DIR or KERNEL_DIR to /usr/src/linux/dickless would do the trick, but it doesn't. Any ideas ?
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Michael Mounteney