libmpcdec: fix cross build

Error without this fix:
```
checking for working memcmp... no
configure: error: working memcmp is not available.
```

pkgs/by-name/li/libmpcdec/package.nix

···
     sha256 = "1a0jdyga1zfi4wgkg3905y6inghy3s4xfs5m4x7pal08m0llkmab";
   };
 
+  # needed for cross builds
+  configureFlags = [ "ac_cv_func_memcmp_working=yes" ];
+
   meta = {
     description = "Musepack SV7 decoder library";
     platforms = lib.platforms.unix;