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/legacy-homebrew#36712.

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

Formula/node.rb

@@ -19,9 +19,12 @@ class Node < Formula
 
   depends_on :python => :build
   depends_on "pkg-config" => :build
-  depends_on "icu4c" => :recommended
   depends_on "openssl" => :optional
 
+  # https://github.com/joyent/node/issues/7919
+  # https://github.com/Homebrew/homebrew/issues/36681
+  depends_on "icu4c" => :optional
+
   fails_with :llvm do
     build 2326
   end
@@ -114,6 +117,18 @@ def caveats
       EOS
     end
 
+    if build.with? "icu4c"
+      s += <<-EOS.undent
+
+        Please note `icu4c` is built with a newer deployment target than Node and
+        this may cause issues in certain usage. Node itself is built against the
+        outdated `libstdc++` target, which is the root cause. For more information see:
+          https://github.com/joyent/node/issues/7919
+
+        If this is an issue for you, do `brew install node --without-icu4c`.
+      EOS
+    end
+
     s
   end