configure: test and add -Wunreachable-code
configure: test and add -Wunreachable-code

Change-Id: I22a6e95c71ba342569342c59e53043255333e786

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