utils: add link_path_manpages method.

Library/Homebrew/utils.rb

@@ -594,3 +594,26 @@ def truncate_text_to_approximate_size(s, max_bytes, options = {})
   out.encode!("UTF-8")
   out
 end
+
+def link_path_manpages(path, command)
+  return unless (path/"man").exist?
+  conflicts = []
+  (path/"man").find do |src|
+    next if src.directory?
+    dst = HOMEBREW_PREFIX/"share"/src.relative_path_from(path)
+    next if dst.symlink? && src == dst.resolved_path
+    if dst.exist?
+      conflicts << dst
+      next
+    end
+    dst.make_relative_symlink(src)
+  end
+  unless conflicts.empty?
+    onoe <<-EOS.undent
+      Could not link #{name} manpages to:
+        #{conflicts.join("\n")}
+
+      Please delete these files and run `#{command}`.
+    EOS
+  end
+end