libomxil-bellagio: fix compiling with llvm

Adds a new check which adds cflags when building with LLVM.

Ref: NixOS#311001

pkgs/development/libraries/libomxil-bellagio/default.nix

@@ -35,6 +35,7 @@ stdenv.mkDerivation rec {
   env.NIX_CFLAGS_COMPILE =
     # stringop-truncation: see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1028978
     if stdenv.cc.isGNU then "-Wno-error=array-bounds -Wno-error=stringop-overflow=8 -Wno-error=stringop-truncation"
+    else if stdenv.cc.bintools.isLLVM && lib.versionAtLeast stdenv.cc.bintools.version "17" then "-Wno-error=unused-but-set-variable"
     else "-Wno-error=absolute-value -Wno-error=enum-conversion -Wno-error=logical-not-parentheses -Wno-error=non-literal-null-conversion";
 
   meta = with lib; {