I've seen, that the google useflag was removed from kdepim-runtime starting with version 17.08.01. Can you add it again or what was the reason for removing it? For testing I've readded it to the ebuild and it compiles fine (without libkgapi).
Upstream has made libkgapi a hard-dependency in 17.12. Many people had issues to get their google accounts set up because they missed the USE flag. So, since it has not future, the USE flag was removed ahead of time.