audit: check GNU urls

We now prefer "ftpmirror.gnu.org" over the main FTP site.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/audit.rb

@@ -220,6 +220,13 @@ def audit_formula_urls f
     end
   end
 
+  # Check GNU urls
+  urls.each do |p|
+    if p =~ %r[ftp\.gnu\.org]
+      problems << " * ftpmirror.gnu.org is preferred for GNU software."
+    end
+  end
+
   return problems
 end