[gtk+] build: Fix vulkan detection

[Matthias Clasen <matthiasc src gnome org>]

commit 9629cbfce4ff60cc611703b367523e77dfb93643
Author: Pavel Grunt <pgrunt redhat com>
Date:   Mon Jan 2 21:46:22 2017 +0100

    build: Fix vulkan detection
    
    Add missing 'test'
    
    https://bugzilla.gnome.org/show_bug.cgi?id=776736

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index a521843..b70a89a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1327,7 +1327,7 @@ if test "x$enable_vulkan" != "xno"; then
           [AC_CHECK_LIB([vulkan], [vkCreateInstance], [], [vulkan_error="Vulkan library not found"])],
           [vulkan_error="Vulkan header not found"])
       if test "x$vulkan_error" != "x"; then
-          if "x$enable_vulkan" = "xyes"; then
+          if test "x$enable_vulkan" = "xyes"; then
               AC_MSG_ERROR($vulkan_error)
           else
               AC_MSG_WARN($vulkan_error)