configure: test for -Wconstant-conversion
configure: test for -Wconstant-conversion

Change-Id: Idf442e35dd8b414c30b8f53ade6f32dfa6f74d39

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