XTerm 397 added support for selecting a rectangular block of text
(https://invisible-island.net/xterm/xterm.log.html#xterm_397).

I have tested the patch and found no problems. Though it is a little
crude, it is a welcome feature.

This is already included, all we need is to enable the build flag as I
have done in here with `--enable-block-select`.

archlinux/devtools!109 (comment 74475)