cmake: use correct libpng target + don't disable libpng versioning

CMakeLists.txt

@@ -356,6 +356,10 @@ if(SDL2IMAGE_ZLIB)
         else()
             set(ZLIB_LIBRARY zlibstatic)
         endif()
+        # ZLIB_INCLUDE_DIRS variable is used by vendored libpng
+        set(ZLIB_INCLUDE_DIRS "${ZLIB_INCLUDE_DIR}")
+        # ZLIB_LIBRARIES variable is used by vendored libpng
+        set(ZLIB_LIBRARIES "${ZLIB_LIBRARY}")
         list(APPEND INSTALL_EXTRA_TARGETS ${ZLIB_LIBRARY})
         set_target_properties(${ZLIB_LIBRARY} PROPERTIES EXPORT_NAME external_zlib)
         add_library(SDL2_image::external_zlib ALIAS ${ZLIB_LIBRARY})
@@ -553,13 +557,12 @@ if(SDL2IMAGE_PNG)
         SDL_IMAGE_SAVE_PNG=$<BOOL:${SDL2IMAGE_PNG_SAVE}>
     )
     if(NOT SDL2IMAGE_BACKEND_STB AND NOT SDL2IMAGE_BACKEND_WIC AND NOT SDL2IMAGE_BACKEND_IMAGEIO)
-        set(HAVE_LD_VERSION_SCRIPT OFF CACHE BOOL "" FORCE)
         if(SDL2IMAGE_PNG_VENDORED)
             message(STATUS "${PROJECT_NAME}: Using vendored libpng")
             sdl_check_project_in_subfolder(external/libpng libpng SDL2IMAGE_VENDORED)
             add_subdirectory(external/libpng EXCLUDE_FROM_ALL)
             if(SDL2IMAGE_PNG_SHARED)
-                set(PNG_LIBRARY png)
+                set(PNG_LIBRARY png_shared)
             else()
                 set(PNG_LIBRARY png_static)
             endif()