configure: simplify libwebp checks

The libwebpmux pkg-config file already has an explicit dependecy on libwebp >= 0.2.0.
Also remove the warning and silently disable the anim encoder when libwebpmux is not new enough.
This is more in line with other library components, like libvpx-vp9

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>
