grantletheme: fix loading in kmail/kontact

This fixes a 5 year old bug (old enough to go to school) where kmail was
not able show emails (also appeared in kontact).

fixes https://github.com/NixOS/nixpkgs/issues/160599

The solution is not pretty but effective and involves
working around the broken search path of kde related libraries.

pkgs/applications/kde/grantleetheme/default.nix

@@ -19,5 +19,12 @@ mkDerivation {
   postInstall = ''
     # added as an include directory by cmake files and fails to compile if it's missing
     mkdir -p "$dev/include/KF5"
+
+    # This is a really disgusting hack, no idea how search paths work for kde,
+    # but apparently kde is looking in $out/$out rather than $out for this library.
+    # Having this symlink fixes kmail finding it and makes my html work (Yay!).
+    mkdir -p $out/$out/lib/grantlee/
+    libpath=$(echo $out/lib/grantlee/*)
+    ln -s $libpath $out/$out/lib/grantlee/$(basename $libpath)
   '';
 }