meson: don't require 'touch' binary, make use of 'cp' optional

meson: don't require 'touch' binary, make use of 'cp' optional

We already didn't use touch (some earlier version of the meson build did ),
and cp is only used for updating unicode files. The latter already depends on
the optional availability of 'wget', so doing the same for 'cp' makes sense.

Eventually we probably want a portable command for updating source code as
part of a target, but for now...

Reported-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/70e96c34-64ee-e549-8c4a-f91a7a668804@dunslane.net