They are sub-libraries of libwebp, enabled by build options
(--enable-libwebp(de)?mux), and therefore I don't think it makes sense
to have separate version prerequisites.

Also let's improve the config output by testing for libwebp(de)?mux only
when libwebp check is successful and displaying more accurate info
(instead of saying "WebP not found", "WebP not built with libwebpmux" is
much more accurate for instance, in order to understand the problem if
you are sure you installed the proper version of libwebp).