[geary] CMakeLists.txt: fix GTK_3_20 detection

[Michael Gratton <mjog src gnome org>]

commit a8930e51d394f549161bd4b1057820ac99bc980b
Author: Gautier Pelloux-Prayer <gautier+git damsy net>
Date:   Tue Oct 4 10:31:26 2016 +0200

    CMakeLists.txt: fix GTK_3_20 detection

 src/CMakeLists.txt |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 486ff6e..0e4ad0b 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -557,7 +557,7 @@ else ()
     message(STATUS "Reference tracking: OFF")
 endif ()
 
-if (GTK3_FOUND AND NOT GTK3_VERSION VERSION_LESS 3.20)
+if (NOT DEPS_gtk+-3.0_VERSION VERSION_LESS 3.20)
     set(EXTRA_VALA_OPTIONS
         ${EXTRA_VALA_OPTIONS}
         -D GTK_3_20