a52dec: fix Darwin build by setting -std=gnu17

Refs: https://github.com/NixOS/nixpkgs/issues/511329

pkgs/by-name/a5/a52dec/package.nix

@@ -23,6 +23,8 @@ stdenv.mkDerivation (finalAttrs: {
     "--enable-shared"
     # Define inline as __attribute__ ((__always_inline__))
     "ac_cv_c_inline=yes"
+    # C23 (Clang default w/ autoconf 2.73) rejects K&R-style declarations
+    "CFLAGS=-std=gnu17"
   ];
 
   makeFlags = [ "AR=${stdenv.cc.targetPrefix}ar" ];