cmake: fix webpmux lib name for cmake linking

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 fixable by renaming `libwebpmux` to `webpmux` in a few places.

This was identified after an update to 1.2.1 of libwebp in the OpenWrt
project:
  https://github.com/openwrt/packages/pull/16766

Internally in OpenWrt, this was patched here:
  https://github.com/openwrt/packages/pull/16784

Change-Id: I97501c62b1e97b133a62b04d6516261d6d7c7fd0
Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
1 file changed