[gdk-pixbuf] build: Fix --without-libtiff not having an effect

[Bastien Nocera <hadess src gnome org>]

commit 5c4edf2cc22468a8410c19cb8b236b227fca300a
Author: Alexander Kanavin <alex kanavin gmail com>
Date:   Tue Oct 10 14:33:18 2017 +0300

    build: Fix --without-libtiff not having an effect
    
    https://bugzilla.gnome.org/show_bug.cgi?id=788770

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index b45f407..9f5d6fb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -676,7 +676,7 @@ if test x$gio_can_sniff = x; then
   AC_DEFINE(GDK_PIXBUF_USE_GIO_MIME, 1, [Define if gio can sniff image data])
 fi
 
-AM_CONDITIONAL(HAVE_TIFF, test "x$libtiff_found" != x)
+AM_CONDITIONAL(HAVE_TIFF, test "x$libtiff_found" != xno)
 AM_CONDITIONAL(HAVE_PNG, test "x$LIBPNG" != x)
 AM_CONDITIONAL(HAVE_JPEG, test "x$LIBJPEG" != x)
 AM_CONDITIONAL(HAVE_JASPER, test "x$LIBJASPER" != x)