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/configure.ac b/src/configure.ac
index b5f217c..86f70f5 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -567,6 +567,16 @@
   AC_MSG_RESULT(yes)
 fi
 
+AC_MSG_CHECKING([diff feature])
+if test "x$features" = "xtiny" -o "x$features" = "xsmall"; then
+  AC_MSG_RESULT([disabled in $features version])
+else
+  AC_MSG_RESULT(enabled)
+  AC_DEFINE(FEAT_DIFF)
+  XDIFF_OBJS_USED="\$(XDIFF_OBJS)"
+  AC_SUBST(XDIFF_OBJS_USED)
+fi
+
 dnl Check for Lua feature.
 AC_MSG_CHECKING(--enable-luainterp argument)
 AC_ARG_ENABLE(luainterp,