libwebp: set meta.pkgConfigModules

Using the package from the pkgs fixpoint instead of
finalAttrs.finalPackage to mirror the other tests which
also (indirectly) use pkgs.libwebp. Slight disadvantage of this is that
it's not possible to test overridden variants of libwebp this way.

pkgs/by-name/li/libwebp/package.nix

@@ -29,6 +29,8 @@
   opencv,
   python3,
   vips,
+  testers,
+  libwebp,
 }:
 
 stdenv.mkDerivation rec {
@@ -79,6 +81,7 @@ stdenv.mkDerivation rec {
       ;
     inherit (python3.pkgs) pillow imread;
     haskell-webp = haskellPackages.webp;
+    pkg-config = testers.hasPkgConfigModules { package = libwebp; };
   };
 
   meta = with lib; {
@@ -87,5 +90,15 @@ stdenv.mkDerivation rec {
     license = licenses.bsd3;
     platforms = platforms.all;
     maintainers = with maintainers; [ ajs124 ];
+    pkgConfigModules = [
+      # configure_pkg_config() calls for these are unconditional
+      "libwebp"
+      "libwebpdecoder"
+      "libwebpdemux"
+      "libsharpyuv"
+    ]
+    ++ lib.optionals libwebpmuxSupport [
+      "libwebpmux"
+    ];
   };
 }