[geary] Enable fatal warnings for valac.

[Michael Gratton <mjog src gnome org>]

commit a2519ff30e1665f39595a8ddee9180700a16bdb5
Author: Michael James Gratton <mike vee net>
Date:   Mon Jun 20 17:35:27 2016 +1000

    Enable fatal warnings for valac.
    
    Doesn't catch anything at the moment, but good to have in case it does in
    the future.

 src/CMakeLists.txt |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 5335e9e..8bd572a 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -574,6 +574,7 @@ set(VALAC_OPTIONS
     --enable-checking
     --debug
     --enable-deprecated
+    --fatal-warnings
     ${EXTRA_VALA_OPTIONS}
 )