Yeah, Debian/Ubuntu maintainers have weird ideas about sonames. This incantation should do it:
sudo ln -s /usr/lib/x86_64-linux-gnu/libSDL2-2.0.so.0 /usr/lib/libSDL2.so
sudo ln -s /usr/lib/x86_64-linux-gnu/libSDL2_image-2.0.so.0 /usr/lib/libSDL2_image.so
sudo ln -s /usr/lib/x86_64-linux-gnu/libSDL2_mixer-2.0.so.0 /usr/lib/libSDL2_mixer.so
sudo ln -s /usr/lib/x86_64-linux-gnu/libSDL2_ttf-2.0.so.0 /usr/lib/libSDL2_ttf.so