[geary] Add a note about vala pre-processor symbol definitions.

[Michael Gratton <mjog src gnome org>]

commit 2ce6d98c53c7b3f2c23cd5250dd5ee79c1be4374
Author: Michael James Gratton <mike vee net>
Date:   Thu Dec 14 09:44:19 2017 +1100

    Add a note about vala pre-processor symbol definitions.

 src/meson.build |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/meson.build b/src/meson.build
index 5f307f3..75da996 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -13,6 +13,8 @@ geary_vala_options = [
   '--enable-deprecated',
 ]
 
+# Symbols for valac's preprocessor must be defined as compiler args,
+# not in the code or in config.h
 if reference_tracking
   geary_vala_options += [ '--define=REF_TRACKING' ]
 endif