Adding the ~x86-macos keyword to cscope-15.6-r1.ebuild is all that is needed for it to install and run properly as a standalone tool and within vim. I was unable to get emacs to build successfully in order to test the emacs USE flag. I am using OS X version 10.4.8 with portage version "Portage 2.1.20.6152 (default-darwin/macos/10.4/x86, gcc-4.0.1, unavailable, 8.8.1 i386)."
~x86-macos in cvs. Thanks!