When I try to compile app-emulation/virtualbox-modules on an amd64 system, it fails, complaining that linux/autoconf.h doesn't exist. This does not happen on my x86 system. When compiling manually, copying /usr/src/linux/include/generated/autoconf.h to each module's linux directory allows it to compile. Reproducible: Always Steps to Reproduce:
Created attachment 227267 [details] app-emulation/virtualbox-modules-3.0.12 output
That changed in kernel 2.6.33 if I remember correctly, a fix is in vbox 3.1.4 and later. I don't intend to backport them and hope to have 3.1.{4,6} stabilized soon.