Should export something like: CMAKE_BINARY_DIR="${WORKDIR}"/${PN}_build For example: One needs to place files in the bin dir that cmake would otherwise fetch by itself. Here is an example from the sunrise overlay where this is used: http://overlays.gentoo.org/proj/sunrise/browser/sunrise/net-irc/irssi-otr/irssi-otr-0.1.ebuild
Kde-testing overlay has the eclass that supports this feature. In week or so it will get to the main tree :]