Fix brew doctor when no Xcode is detected.

Library/Homebrew/cmd/doctor.rb

@@ -194,7 +194,13 @@ def check_gcc_versions
     EOS
   end
 
-  if MacOS.xcode_version < "4.0"
+  if MacOS.xcode_version == nil
+      puts <<-EOS.undent
+        We couldn't detect any version of Xcode.
+        If you downloaded Xcode 4.1 from the App Store, you may need to run the installer.
+
+      EOS
+  elsif MacOS.xcode_version < "4.0"
     if gcc_40 == nil
       puts <<-EOS.undent
         We couldn't detect gcc 4.0.x. Some formulae require this compiler.