warn if autorot + no exif

warn if you use the --rotate option to vipsthumbnail but vips has been
built without libexif

see https://github.com/jcupitt/libvips/issues/319

tools/vipsthumbnail.c

@@ -68,6 +68,8 @@
  * 9/5/15
  * 	- use vips_resize() instead of our own code
  * 	- premultiply alpha
+ * 30/7/15
+ * 	- warn if you autorot and there's no exif support
  */
 
 #ifdef HAVE_CONFIG_H
@@ -720,6 +722,14 @@ main( int argc, char **argv )
 		thumbnail_height = thumbnail_width;
 	}
 
+	if( rotate_image ) {
+#ifndef HAVE_EXIF
+		vips_warn( "vipsthumbnail", "%s",
+			_( "auto-rotate disabled: "
+			      "libvips built without exif support" ) );
+#endif /*!HAVE_EXIF*/
+	}
+
 	result = 0;
 
 	for( i = 1; i < argc; i++ ) {