patch 8.2.4039: the xdiff library is linked in even when not used

Problem:    The xdiff library is linked in even when not used.
Solution:   Use configure to decide whether xdiff object files are included.
diff --git a/src/auto/configure b/src/auto/configure
index cde05d6..72d2d07 100755
--- a/src/auto/configure
+++ b/src/auto/configure
@@ -714,6 +714,7 @@
 vi_cv_path_plain_lua
 vi_cv_path_luajit
 vi_cv_path_lua
+XDIFF_OBJS_USED
 compiledby
 dogvimdiff
 dovimdiff
@@ -5305,6 +5306,20 @@
 $as_echo "yes" >&6; }
 fi
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking diff feature" >&5
+$as_echo_n "checking diff feature... " >&6; }
+if test "x$features" = "xtiny" -o "x$features" = "xsmall"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: disabled in $features version" >&5
+$as_echo "disabled in $features version" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: enabled" >&5
+$as_echo "enabled" >&6; }
+  $as_echo "#define FEAT_DIFF 1" >>confdefs.h
+
+  XDIFF_OBJS_USED="\$(XDIFF_OBJS)"
+
+fi
+
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking --enable-luainterp argument" >&5
 $as_echo_n "checking --enable-luainterp argument... " >&6; }
 # Check whether --enable-luainterp was given.