Changed check so -define tiff:ignore-layers also works.
diff --git a/coders/tiff.c b/coders/tiff.c
index 46a7469..bb4ef87 100644
--- a/coders/tiff.c
+++ b/coders/tiff.c
@@ -965,7 +965,7 @@
   if (GetImageListLength(image) != 1)
     return;
   option=GetImageOption(image_info,"tiff:ignore-layers");
-  if (IsStringTrue(option) != MagickFalse)
+  if (option != (const char * ) NULL)
     return;
   layer_info=GetImageProfile(image,"tiff:37724");
   if (layer_info == (const StringInfo *) NULL)