unsigned_kext: use binary and cask options now.

Library/Homebrew/requirements/unsigned_kext_requirement.rb

@@ -5,12 +5,35 @@ class UnsignedKextRequirement < Requirement
 
   satisfy { MacOS.version < :yosemite }
 
+  def initialize(tags=[])
+    tags.each do |tag|
+      next unless tag.is_a? Hash
+      @binary ||= tag[:binary]
+      @cask ||= tag[:cask]
+    end
+    super
+  end
+
   def message
-    <<-EOS.undent
-      OS X Mavericks or older is required for this package.
-      OS X Yosemite introduced a strict unsigned kext ban which breaks this package.
-      You should remove this package from your system and attempt to find upstream
-      binaries to use instead.
+    s = <<-EOS.undent
+      Building this formula from source isn't possible due to OS X
+      Yosemite and above's strict unsigned kext ban.
+    EOS
+
+    if @cask
+      s +=  <<-EOS.undent
+
+        You can install from Homebrew Cask:
+          brew install Caskroom/cask/#{@cask}
+      EOS
+    end
+
+    if @binary
+      s += <<-EOS.undent
+
+        You can use the upstream binary:
+          #{@binary}
       EOS
     end
   end
+end