meson: Add toggle for glx-direct

GNU Hurd needs to turn off glx-direct, rather than special case it,
we'll just add a toggle.

CC: 18.3 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/meson_options.txt b/meson_options.txt
index a1d5ab0..589d10b 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -318,3 +318,9 @@
   choices : ['auto', 'true', 'false'],
   description : 'Enable VK_EXT_acquire_xlib_display.'
 )
+option(
+  'glx-direct',
+  type : 'boolean',
+  value : true,
+  description : 'Enable direct rendering in GLX and EGL for DRI',
+)