meson: set apple glx defines

Reviewed-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/meson.build b/meson.build
index d6ffa30..030ab0c 100644
--- a/meson.build
+++ b/meson.build
@@ -617,6 +617,8 @@
     endif
     if with_dri_platform == 'drm'
       pre_args += '-DGLX_USE_DRM'
+    elif with_dri_platform == 'apple'
+      pre_args += '-DGLX_USE_APPLEGL'
     elif with_dri_platform == 'windows'
       pre_args += '-DGLX_USE_WINDOWSGL'
     endif