Remove the equivalent of sdl2.m4

[icculus]

Remove the equivalent of sdl2.m4 (AM_PATH_SDL2), document PKG_CHECK_MODULES([SDL3], [sdl3]) as the recommendation for Autotools users.

CMake users should continue to use find_package(SDL3) and SDL3::SDL3 (or SDL3::SDL3-static), no change there except for replacing 2 with 3.

Remove the equivalent of sdl2-config, document pkg-config sdl3 as the direct replacement (but Autotools users should prefer PKG_CHECK_MODULES, and artisanal hand-crafted makefiles/scripts should check $PKG_CONFIG and use that if set, falling back to pkg-config if unset).

Originally posted by @smcv in #3519 (comment)

(This might be in progress on other open issues, in which case just close this, I'm just splitting out the "Ideas for SDL 3" megathread. --ryan.)