configure: add -Wshorten-64-to-32
configure: add -Wshorten-64-to-32

adds a visual studio like warning about implicit conversions from 64 to
32-bits

Change-Id: I8a984ec6f61bd65f0c71756f3e1ee89a68fed852

Aexyn/libwebp,PKRoma/libwebp,yohunl/libwebp,ttyangf/libwebp,PKRoma/libwebp,yohunl/libwebp,ttyangf/libwebp,yohunl/libwebp,redengineer/libwebp,yohunl/libwebp,ttyangf/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,Aexyn/libwebp,ttyangf/libwebp,PKRoma/libwebp,PKRoma/libwebp,ttyangf/libwebp,redengineer/libwebp,Aexyn/libwebp,yohunl/libwebp,webmproject/libwebp,PKRoma/libwebp,Aexyn/libwebp,redengineer/libwebp,redengineer/libwebp,redengineer/libwebp,Aexyn/libwebp