I've created a simple patch on kde_src_compile in kde.eclass to inform the user about the DO_NOT_COMPILE environment. This is usefull for users who don't want all applications included in the source tarball.
Created attachment 36607 [details, diff] kde.eclass.patch
Maybe we'll be able to break up kde-base packages in multiple ebuilds with correct dependencies at some point, but DO_NOT_COMPILE is not reflected via R/DEPEND variables and therefore I consider it as not officially supported. If it breaks something, you're on your own.