Well, the build system is a bit confusing and I can't really tell how complete the port is, still it seems to work. You need to pass SDL_CONFIG=sdl2-config to configure.
commit 14aa965fafaadfdc2cd5ae70934edbb40091a0e6 Author: Lars Wendler <polynomial-c@gentoo.org> Date: Thu Nov 10 16:11:39 2016 games-engines/scummvm: Use libsdl2 (bug #599394). Bump to EAPI-6. Package-Manager: portage-2.3.2