externals: Use SDL2 statically

Building it as a shared library causes issues distributing it to an
AppImage, since linuxdeploy expects the executable to only dynamically
link to system libraries. Additionally, simply dynamically linking to a
library in the binary directory is bound to cause issues.

Solution is to use SDL's CMake switches and build it statically. We also
alias `SDL2` to `SDL2-static` on the external submodule for
compatibility with the rest of the project.