cmake: add support for webpmux
cmake: add support for webpmux

Change-Id: Ia694ba49549c43956f6552e6981a46df0fe7e686

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