libsForQt5.kjs: remove pcre dependency

pcre is actually still in use by kjs.
Removing it runs the risk of subtle breakages, see also the upstream notice [1].
However, `pcre` 1.x is insecure and unmaintained and should be removed.
With plasma 5 being scheduled for removal,
the breakages caused by removing pcre from kjs should be non-critical.

[1] 3c663ad8ac/CMakeLists.txt (L36-46)

pkgs/development/libraries/kde-frameworks/kjs.nix

@@ -1,8 +1,8 @@
 {
+  lib,
   mkDerivation,
   extra-cmake-modules,
   kdoctools,
-  pcre,
   qtbase,
 }:
 
@@ -13,7 +13,12 @@ mkDerivation {
     kdoctools
   ];
   buildInputs = [
-    pcre
     qtbase
   ];
+  cmakeFlags = [
+    # this can break stuff, see:
+    # https://invent.kde.org/frameworks/kjs/-/blob/3c663ad8ac16f8982784a5ebd5d9200e7aa07936/CMakeLists.txt#L36-46
+    # However: It shouldn't break much considering plasma 5 is planned to be removed.
+    (lib.cmakeBool "KJS_FORCE_DISABLE_PCRE" true)
+  ];
 }