cmake: fix builtin detection w/-Werror
cmake: fix builtin detection w/-Werror

and other warnings enabled (-Wunused-value)

Change-Id: Ie3605628b4cbfb5bfd1e847cc504cb96f2825388

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