Which licenses portage accepts is governed by the `ACCEPT_LICENSE` variable in `make.conf`. This variable is the counterpart to `package.license` the same way `USE` is to `package.use`, `ACCEPT_KEYWORDS` to `package.keywords`, etc.. By default, this variable is set to `@FREE`, which means "only accept all licenses in the `FREE` set". This set contains all licenses that require the user to accept them. In order to be able to install all packages without additional license-related prompts, just add the following line to `/etc/portage/make.conf`:
ACCEPT_LICENSE="*"
Conversely, you could also block out all non-free packages with `ACCEPT_LICENSE="@FREE"`, or choose any other combination of licenses you like. See `/usr/portage/profiles/license_groups` for details