meson-winbuild.sh: sdl2-client build support
Though there is no hard dependency, it's probably better to have #43923 resolved before this.
Touched parts of the script also go from "x$abc" -type comparisons to plain "$abc". There's no need to use former in 2022 (as long as there's "" to make sure shell knows there's parameter there). I'm about to use latter style in all new shell scripting.
As meson build's linking against sdl2 libraries on Windows has been resolved on #43776, we should be able to add sdl2-client build support to meson-winbuild.sh