Version 0.6.1 of gemrb was just released. The preferred build system is now cmake. This deliberately drops creating the cache directory. The cache directory cannot be shared between multiple running instances. Tested with i686/x86_64. I am one of the upstream developers.
Created attachment 235599 [details] gemrb v0.6.1 ebuild
gemrb-0.6.2.ebuild is in portage. Thanks for the bug report and ebuild submission.