[SDL2] fixes to autotools find_lib usage for mac (and windows)

[sezero]

Before this, configure used to find dylibs like this:

-- dynamic libwebp -> libwebp.dylib
-- dynamic libwebpdemux -> libwebpdemux.dylib
-- dynamic libavif -> libavif.dylib
-- dynamic libjxl -> libjxl.dylib

However, libXXX.dylib are most usually symlinks to their versioned ones
i.e. libXXX.N.dylib, therefore it was the wrong thing to do.

After this, it finds the dylibs like this: (configure script was
deliberately run with --disable-stb-image --disable-imageio in order
to find all possible dylibs) :

-- dynamic libwebp -> libwebp.7.dylib
-- dynamic libwebpdemux -> libwebpdemux.2.dylib
-- dynamic libavif -> libavif.16.dylib
-- dynamic libtiff -> libtiff.5.dylib
-- dynamic libjpeg -> libjpeg.9.dylib
-- dynamic libpng -> libpng16.dylib
-- dynamic libjxl -> libjxl.0.8.dylib

As for windows: Major offender was libavif because of the same versioning
issue -- now fixed.

@slouken, @madebr: Please review, if you can.

P.S.: Actually, SDL2_mixer has the same issue. Maybe I'd apply similar
changes there after this.

[sezero]

Merged.

[sezero]

Pushed libsdl-org/SDL_mixer@5885b5e to SDL_mixer SDL2 branch