When using CMake the `<prefix>/usr/share/WebP/cmake/WebPConfig.cmake` gets
used to get the names of the libraries to link against.

Since version 1.2.1 of libwebp, libwebpmux is on by default.
This causes a linker error because the linker arg should be `-lwebpmux`
instead of `-llibwebpmux`.

This is easily fixable by correcting the generation of the WebPConfig.cmake
file to append `webpmux` when this lib is enabled.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>