In order to have flowtop, you must have compiled ncurses with the static flag - perhaps provide a USE flag in netsniff-ng that will require ncurses to be built appropriately?
Sorry, I cannot reproduce this: my logfile: http://b-4.xmw.de/var/log/portage/build/net-analyzer/netsniff-ng-0.5.8_rc2:20130808-111650.log # emerge -pv ncurses These are the packages that would be merged, in order: Calculating dependencies... done! [ebuild R ] sys-libs/ncurses-5.9-r3:5 USE="cxx gpm unicode -ada -debug -doc -minimal -profile -static-libs -tinfo -trace" ABI_X86="32 (64) (-x32)" 0 kB