libmodplug: fix build with clang 16

Remove usage of the register storage class, which is not allowed in
C++17 (the default for clang 16).

pkgs/development/libraries/libmodplug/default.nix

@@ -9,6 +9,11 @@ stdenv.mkDerivation rec {
     sha256 = "1pnri98a603xk47smnxr551svbmgbzcw018mq1k6srbrq6kaaz25";
   };
 
+  # Unfortunately, upstream appears inactive and the patches from the fork don’t apply cleanly.
+  # Modify `src/fastmix.cpp` to remove usage of the register storage class, which is
+  # not allowed in C++17 and is an error in clang 16.
+  prePatch = "substituteInPlace src/fastmix.cpp --replace 'register ' ''";
+
   outputs = [ "out" "dev" ];
 
   preConfigure = ''