hwloc: disable x11 by default

I think nothing actually uses what this feature provides but all headless
machines suffer.

pkgs/development/libraries/hwloc/default.nix

@@ -1,8 +1,9 @@
 { stdenv, fetchurl, pkgconfig, expat, ncurses, pciutils, numactl
-, cairo, libX11
-, x11Support ? (!stdenv.isCygwin)
+, x11Support ? false, libX11 ? null, cairo ? null
 }:
 
+assert x11Support -> libX11 != null && cairo != null;
+
 with stdenv.lib;
 
 stdenv.mkDerivation rec {