configure: restore 2 warnings
configure: restore 2 warnings

-Wshorten-64-to-32 and -Wunreachable-code were lost in:
96201e5 migrate anim_diff tool from C++ to C89

Change-Id: Ic7161908b1aa88094563f0c7bcad1d508101d015

ttyangf/libwebp,ttyangf/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp