Don't require cmake files when using system webp and webm, use PkgConfig instead #1399
Description
libwebp and libwebm system libraries can used instead of the bundled/vendor copies, which is great.
However, the CMake files must be installed on the system to use the system libraries, and those files are not always installed. Specifically, libwebp can be built using autotools or CMake, and since the CMake files are only installed when using CMake , if libwebp is built using autotools, no cmake files are installed.
For example, Gentoo doesn't install the CMake files for webp because it uses autotools for this package. See https://bugs.gentoo.org/937031 for details.
The standard way to use libraries is with PkgConfig - this approach will always work. For libwebp: pkg_check_modules(WebP REQUIRED libwebp)
FYI @wbruna