Same mistake I made a while ago. :)
The ./configure script for SDL2 should show what libraries/features are compiled into SDL2 at the end. I believe I had missed some important X11/OpenGL packages. Instead of failing, the build produced a flawed SDL2 shared library without all targets built in.
I really recommend getting the library from the Steam Runtime, but if you'd rather build yourself, see the dependencies here:
https://hg.libsdl.org/SDL/file/default/docs/README...