node: tweak icu4c setup
node: tweak icu4c setup

Personally, I’d be content to keep the dep as recommended, but it’s
possible the big red warning on the different build standards is making
people jumpy, and there are use cases where the `icu4c` dep does
interfere with certain usage, so I’ve partially reverted the direction
I took in the Node PR the other day by making the new ICU capabilities
optional.

Also added a caveat for the benefit of clarity, and hope more people
complain to Node upstream to fix the real root cause of the problem
here and make life for packagers a little easier.

Closes Homebrew/homebrew#36712.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

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