hm: apply upstream patch for gcc 16

GCC 16 has better analysis of unused variables than previous versions.
Since HM builds with -Werror by default, this causes a build failure, so
we fetch the commit that fixed this issue upstream.

We elected to take a patch instead of suppressing the error (with
-Wno-error=…) since patches will be removed on the next update while
flags often sit untouched and unnoticed long after they're necessary.

pkgs/by-name/hm/hm/package.nix

@@ -20,6 +20,18 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-zWBwrnCNKi2sIopdu2XQj/7IoTsJQzlcIFNNKM0glDQ=";
   };
 
+  patches = [
+    # GCC 16 has better analysis of unused variables than previous versions.
+    # Since HM builds with -Werror by default, this causes a build failure, so
+    # we fetch the commit that fixed this issue upstream until it makes it into
+    # a release.
+    (fetchpatch {
+      name = "gcc16-unused-variable.patch";
+      url = "https://vcgit.hhi.fraunhofer.de/jvet/HM/-/commit/884d704b9d04ac1728b2c6049174e47eb04987d2.patch";
+      hash = "sha256-UqZf7XFTEOpfE0Uyyrq+ydScR9GzMgR+JQVeZGFjc1c=";
+    })
+  ];
+
   postPatch = ''
     substituteInPlace CMakeLists.txt \
       --replace-fail "-msse4.1" ""