Switched to profile 17.0 and followed all instructions according to news item "New 17.0 profiles in the Gentoo repository". During world rebuild, kbuild fails to build.
Created attachment 509784 [details] Output of 'emerge --info =dev-util/kbuild-0.1.9998_pre20131130-r1::gentoo' Output of: # emerge --info '=dev-util/kbuild-0.1.9998_pre20131130-r1::gentoo'
Created attachment 509786 [details] Build log
Presumably related to Bug 640912 (app-emulation/virtualbox). Also tried dev-util/kbuild-0.1.9998_pre20171020, fails with mostly the same errors.
This issue was caused by (now ancient) ccache versions. 3.2.8+ and 3.3.1+ fixed that respectively. No ccache version older than 3.7 currently exists in tree, so I guess this bug report can be closed.