blacklist: port to generic layer.

Library/Homebrew/blacklist.rb

@@ -28,16 +28,6 @@ def blacklisted?(name)
     EOS
   when /(lib)?lzma/
     "lzma is now part of the xz formula."
-  when "xcode"
-    if MacOS.version >= :lion
-      <<-EOS.undent
-      Xcode can be installed from the App Store.
-      EOS
-    else
-      <<-EOS.undent
-      Xcode can be installed from https://developer.apple.com/xcode/downloads/
-      EOS
-    end
   when "gtest", "googletest", "google-test" then <<-EOS.undent
     Installing gtest system-wide is not recommended; it should be vendored
     in your projects that use it.
@@ -97,3 +87,6 @@ def blacklisted?(name)
     EOS
   end
 end
+alias generic_blacklisted? blacklisted?
+
+require "extend/os/blacklist"

Library/Homebrew/extend/os/blacklist.rb

@@ -0,0 +1,5 @@
+require "blacklist"
+
+if OS.mac?
+  require "extend/os/mac/blacklist"
+end

Library/Homebrew/extend/os/mac/blacklist.rb

@@ -0,0 +1,16 @@
+def blacklisted?(name)
+  case name.downcase
+  when "xcode"
+    if MacOS.version >= :lion
+      <<-EOS.undent
+      Xcode can be installed from the App Store.
+      EOS
+    else
+      <<-EOS.undent
+      Xcode can be installed from https://developer.apple.com/xcode/downloads/
+      EOS
+    end
+  else
+    generic_blacklisted?(name)
+  end
+end

Library/Homebrew/test/test_blacklist.rb

@@ -30,10 +30,6 @@ class BlacklistTests < Homebrew::TestCase
     %w[lzma liblzma].each { |s| assert_blacklisted s }
   end
 
-  def test_xcode
-    %w[xcode Xcode].each { |s| assert_blacklisted s }
-  end
-
   def test_gtest
     %w[gtest googletest google-test].each { |s| assert_blacklisted s }
   end

Library/Homebrew/test/test_os_mac_blacklist.rb

@@ -0,0 +1,12 @@
+require "testing_env"
+require "blacklist"
+
+class BlacklistTests < Homebrew::TestCase
+  def assert_blacklisted(s)
+    assert blacklisted?(s), "'#{s}' should be blacklisted"
+  end
+
+  def test_xcode
+    %w[xcode Xcode].each { |s| assert_blacklisted s }
+  end
+end