qt5.qtwebengine: fix Qt version number in cmake

This fixes the build failures in packages depending on qtwebengine and
using cmake to find the Qt dependencies (like libsForQt5.messagelib).

pkgs/development/libraries/qt-5/5.15/default.nix

···
       qtvirtualkeyboard = callPackage ../modules/qtvirtualkeyboard.nix {};
       qtwayland = callPackage ../modules/qtwayland.nix {};
       qtwebchannel = callPackage ../modules/qtwebchannel.nix {};
-      qtwebengine = callPackage ../modules/qtwebengine.nix {};
+      qtwebengine = callPackage ../modules/qtwebengine.nix {
+        inherit (srcs.qtwebengine) version;
+      };
       qtwebglplugin = callPackage ../modules/qtwebglplugin.nix {};
       qtwebkit = callPackage ../modules/qtwebkit.nix {};
       qtwebsockets = callPackage ../modules/qtwebsockets.nix {};

pkgs/development/libraries/qt-5/modules/qtwebengine.nix

···
 , cups, darwin, openbsm, runCommand, xcbuild, writeScriptBin
 , ffmpeg_3 ? null
 , lib, stdenv, fetchpatch
+, version ? null
 , qtCompatVersion
 }:
 
···
     [Paths]
     Prefix = ..
     EOF
+  '' + lib.optionalString (lib.versions.majorMinor qtCompatVersion == "5.15") ''
+    # Fix for out-of-sync QtWebEngine and Qt releases (since 5.15.3)
+    sed 's/${lib.head (lib.splitString "-" version)} /${qtCompatVersion} /' -i "$out"/lib/cmake/*/*Config.cmake
   '';
 
   meta = with lib; {