* gnu/packages/sdl.scm (sdl3): Update to 3.2.8.
[arguments]: Add additional flags for using shared libraries.
[propagated-inputs]: Add all inputs needed by pkg-config at configure time.
[inputs]: Add all other inputs, copied from SDL2.
Change-Id: Ia18dc5867e19ca6c4e265c596d6f769549fe05d8