better im_openout() compat

ChangeLog

@@ -2,6 +2,7 @@
 - fix vips_rawsave_fd(), thanks aferrero2707 
 - fix im_point()
 - vips_scale() now does round to nearest to avoid rounding errors
+- improve im_openout() compat macro
 
 30/6/14 started 7.40.3
 - fix interlaced thumbnails in vipsthumbnail, thanks lovell

libvips/include/vips/vips7compat.h

@@ -225,6 +225,12 @@ int im_demand_hint (IMAGE * im, VipsDemandStyle hint, ...);
 #define im_invalidate vips_image_invalidate_all
 #define im_isfile vips_image_isfile
 #define im_printdesc( I ) vips_object_print_dump( VIPS_OBJECT( I ) )
+
+/* im_openout() needs to have this visible.
+ */
+VipsImage * 
+vips_image_new_mode( const char *filename, const char *mode );
+
 #define im_openout( F ) vips_image_new_mode( F, "w" )
 #define im_setbuf( F ) vips_image_new( "t" )