build_config: fix `SDL_DEFAULT_ASSERT_LEVEL`
Currently, `SDL_DEFAULT_ASSERT_LEVEL` is commented out by CMake when its value is 0, setting the assertions level to the default value instead of disabling them.
This change:
- defines `SDL_DEFAULT_ASSERT_LEVEL_CONFIGURED` when its value is non-zero.
- defines `SDL_DEFAULT_ASSERT_LEVEL`, regardless of its value, when `SDL_DEFAULT_ASSERT_LEVEL_CONFIGURED` is defined.
L zard