cmake: skip install logic of libavif and libpng CMake script

CMakeLists.txt

@@ -488,6 +488,7 @@ if(SDL3IMAGE_AVIF)
         set(SDL3IMAGE_AVIF_ENABLED TRUE)
         message(STATUS "${PROJECT_NAME}: Using vendored libavif")
         sdl_check_project_in_subfolder(external/libavif libavif SDL3IMAGE_VENDORED)
+        set(SKIP_INSTALL_LIBRARIES TRUE)
         set(BUILD_SHARED_LIBS ${SDL3IMAGE_AVIF_SHARED})
         set(AVIF_CODEC_DAV1D ON CACHE BOOL "Use dav1d codec for decoding" FORCE)
         set(AVIF_LOCAL_DAV1D OFF CACHE BOOL "Build dav1d by libavif" FORCE)
@@ -721,6 +722,7 @@ if(SDL3IMAGE_PNG)
             set(SDL3IMAGE_PNG_ENABLED TRUE)
             message(STATUS "${PROJECT_NAME}: Using vendored libpng")
             set(PNG_TESTS OFF CACHE BOOL "Build PNG Tests" FORCE)
+            set(SKIP_INSTALL_EXPORT TRUE)
             sdl_check_project_in_subfolder(external/libpng libpng SDL3IMAGE_VENDORED)
             add_subdirectory(external/libpng EXCLUDE_FROM_ALL)
             if(SDL3IMAGE_PNG_SHARED)