[gtk+] Fix the configure check for Vulkan

[Matthias Clasen <matthiasc src gnome org>]

commit 17c8ebc4a8bac95190206744b3712ad9bc1fbc71
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Dec 9 14:00:48 2016 -0500

    Fix the configure check for Vulkan
    
    GDK_WINDOWING_VULKAN was getting defined regardless of the tests.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 8093384..d2c7158 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1325,7 +1325,7 @@ if test "x$enable_vulkan" != "xno"; then
       vulkan_error=""
       AC_CHECK_HEADER([vulkan/vulkan.h], [], [vulkan_error="Vulkan header not found"])
       AC_CHECK_LIB([vulkan], [vkCreateInstance], [], [vulkan_error="Vulkan library not found"])
-      if test "x$vulkan_result" != "x"; then
+      if test "x$vulkan_error" != "x"; then
           if "x$enable_vulkan" = "xyes"; then
               AC_MSG_ERROR($vulkan_error)
           else