Skip standard compilers check for unknown Xcode

Rescuing false on the StandardCompilers map meant that the doctor
check assumed that the compilers were incorrect, not merely
unknown.

Instead, skip the check and return nil for unknown Xcode; nil
should be interpreted as "dunno", instead of "true" or "false",
and the doctor check no longer prints on nil.

Library/Homebrew/cmd/doctor.rb

@@ -259,7 +259,8 @@ end
 
 def check_standard_compilers
   return if check_for_latest_xcode # only check if Xcode is up to date
-  if !MacOS.compilers_standard? then <<-EOS.undent
+  compiler_status = MacOS.compilers_standard?
+  if not compiler_status and not compiler_status.nil? then <<-EOS.undent
     Your compilers are different from the standard versions for your Xcode.
     If you have Xcode 4.3 or newer, you should install the Command Line Tools for
     Xcode from within Xcode's Download preferences.

Library/Homebrew/macos.rb

@@ -204,9 +204,10 @@ module MacOS extend self
 
         Thanks!
         EOS
+      return
     end
 
-    StandardCompilers[xcode].all? { |method, build| MacOS.send(method) == build } rescue false
+    StandardCompilers[xcode].all? { |method, build| MacOS.send(method) == build }
   end
 
   def app_with_bundle_id id