Further work on animated GIF export: XDG Pictures

Using XDG's user dir settings to determine where pictures are
stored for a user (e.g., "~/Pictures" -- used as a fallback).
May be overridden using "--exportdir".

Also, while I was updating some docs, replace references to
"Mac OS X" with "macOS", the new name of that OS these days.

docs/en/INSTALL.txt

@@ -2,11 +2,11 @@ INSTALL.txt for Tux Paint
 
 Tux Paint - A simple drawing program for children.
 
-Copyright (c) 2002-2018
+Copyright (c) 2002-2020
 Various contributors (see below, and AUTHORS.txt)
 http://www.tuxpaint.org/
 
-June 27, 2002 - December 8, 2018
+June 27, 2002 - July 25, 2020
 $Id$
 
 
@@ -119,6 +119,14 @@ Requirements:
         Also depends on:
           libxml2
 
+    Animated GIF Export feature
+    ---------------------------
+      So support export of animated GIFs (slideshows), the
+      "libimagequant" library (from the "pngquant2" project)
+      is required.
+
+        https://github.com/ImageOptim/libimagequant
+
     NetPBM Tools  [OPTIONAL]  [No longer used, by default]
     ------------------------
       Under Linux and Unix, the NetPBM tools are what are currently