patch 9.1.0687: Makefile may not install desktop files

Problem:  Makefile may not install desktop files
Solution: Check for "$(DESTDIR)$(DATADIR)" instead of just "$DESTDIR",
          which is usually not defined, add uninstall rules for the
          icons and the desktop files

closes: #15528
fixes: #15546

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    687,
 /**/
     686,
 /**/