In portage-188.8.131.52 and 2.2.0_alpha133 there’s support for expermental EAPI 5-hdepend which adds the HDEPEND variable which is used to represent build-time host dependencies. For build-time target dependencies, use DEPEND (if the host is the target then both HDEPEND and DEPEND will be installed on it). There’s a special “targetroot” USE flag that will be automatically enabled for packages that are built for installation into a target ROOT, and will otherwise be automatically disabled. This flag may be used to control conditional dependencies, and ebuilds that use this flag need to add it to IUSE unless it happens to be included in the profile’s IUSE_IMPLICIT variable.
For those who may not be familiar with the history of HDEPEND, it was originally suggested in bug #317337. That was in 2010, and later that year there was some discussion about it on the chromium-os-dev mailing list. Recently, I suggested on the gentoo-dev mail list that it be included in EAPI 5, but it didn’t make it in. Since then, there’s been some renewed effort , and now the patch is included in mainline Portage.