CMake: set rpath for shared objects
CMake: set rpath for shared objects

this ensures the correct library is loaded when the install path is not
in the default search path; this matches the behavior of
autoconf/libtool

Change-Id: I7eaa458c353cbdb7f0a1f782a879c59662a5de79

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