fix: Use PkgConfig for WebP and WebM

[candrews]

PkgConfig is the standard approach to locating and using system libraries. It does not rely on the library having been built with CMake, improving compatibility.

webpmux is the library name; it should not be prefixed with lib. See: webmproject/libwebp@bfad7ab

Closes: #1399

[wbruna]

Pretty sure we shouldn't assume pkg-config will always be available, e.g. on Windows. I believe it would be best to call pkg_check_modules as a fallback, in case the CMake module isn't found.

[candrews]

Pretty sure we shouldn't assume pkg-config will always be available, e.g. on Windows. I believe it would be best to call pkg_check_modules as a fallback, in case the CMake module isn't found.

That's a good point. I've done as you suggested, only use PkgConfig if it's available.

[leejet]

This change appears to have broken CI (link error with -lwebpmux). Please fix the build regression first.

Also, I think there are two issues in the current implementation:

1. Fallback logic is still incorrect
   The code only falls back when find_package(PkgConfig) is not found. However, a much more common failure case is when pkg-config exists but the required .pc files are missing or mismatched.
   Using pkg_check_modules(... REQUIRED ...) will fail configuration immediately in that case, so the fallback path is never reached.

2. Missing libwebpmux in pkg-config path
   The pkg-config branch only resolves libwebp, but webpmux is a separate module upstream (libwebpmux.pc).
   So PkgConfig::WebP may only provide -lwebp, while the examples still require webpmux to be resolved and linked separately.

[candrews]

This change appears to have broken CI (link error with -lwebpmux). Please fix the build regression first.

Done! I added an alias so libwebpmux and webpmux both work in all cases. I undid the modifications to examples/{cli,server}.

Also, I think there are two issues in the current implementation:

1. **Fallback logic is still incorrect**
   The code only falls back when `find_package(PkgConfig)` is not found. However, a much more common failure case is when `pkg-config` exists but the required `.pc` files are missing or mismatched.
   Using `pkg_check_modules(... REQUIRED ...)` will fail configuration immediately in that case, so the fallback path is never reached.

I've addressed that by trying the CMake approach first, then the PkgConfig approach. I also used cmake_pkg_config to eliminate the use of the PkgConfig executable and simplify the logic.

2. **Missing `libwebpmux` in pkg-config path**
   The pkg-config branch only resolves `libwebp`, but `webpmux` is a separate module upstream (`libwebpmux.pc`).
   So `PkgConfig::WebP` may only provide `-lwebp`, while the examples still require `webpmux` to be resolved and linked separately.

I added webpmux. This wasn't necessary on my system, but it makes sense to have it, and it still works.

[wbruna]

CMake 4.1 seems to be younger than a year; this would exclude most current Linux distros (e.g. Debian Stable has 3.31.6).