Use CMAKE_CURRENT_*_DIR instead of CMAKE_*_DIR

[humbertodias]

This PR allows for building with cmake's add_subdirectory function, like so:

Build and include SDL in main project

add_subdirectory(lib/SDL2_image lib/SDL2_image_BIN EXCLUDE_FROM_ALL)
target_link_libraries(main PRIVATE SDL2_image)

[sezero]

(i) Same goes for the SDL2 branch

(ii) Same goes for both SDL2 nd SDL3 branches of all other SDL satellite libs

[madebr]

Like the grim reaper, I went around and applied this change to all satellite libraries.
SDL itself already did this.