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.build b/meson.build
index 8eb5c09..57dd8b5 100644
--- a/meson.build
+++ b/meson.build
@@ -51,6 +51,7 @@
 with_libunwind = get_option('libunwind')
 with_asm = get_option('asm')
 with_glx_read_only_text = get_option('glx-read-only-text')
+with_glx_direct = get_option('glx-direct')
 with_osmesa = get_option('osmesa')
 with_swr_arches = get_option('swr-arches')
 with_tools = get_option('tools')
@@ -365,9 +366,6 @@
   endif
 endif
 
-# TODO: toggle for this
-with_glx_direct = true
-
 if with_vulkan_icd_dir == ''
   with_vulkan_icd_dir = join_paths(get_option('datadir'), 'vulkan/icd.d')
 endif