setroot: fix darwin build

pkgs/tools/X11/setroot/default.nix

···
   buildInputs = [ libX11 imlib2 ]
     ++ stdenv.lib.optional enableXinerama libXinerama;
 
-  buildFlags = if enableXinerama then "xinerama=1" else "xinerama=0";
+  buildFlags = "CC=cc " + (if enableXinerama then "xinerama=1" else "xinerama=0");
 
   installFlags = "DESTDIR=$(out) PREFIX=";