doctor: remove gcc-4.2 warning on Xcode 4.2+

Re: Homebrew/homebrew#9925, many others.

Library/Homebrew/cmd/doctor.rb

@@ -182,11 +182,13 @@ def check_gcc_versions
   gcc_40 = MacOS.gcc_40_build_version
 
   if gcc_42 == nil
-    puts <<-EOS.undent
-      We couldn't detect gcc 4.2.x. Some formulae require this compiler.
-      NOTE: Versions of XCode newer than 4.2 don't include gcc 4.2.x.
+    # Don't show this warning on Xcode 4.2+
+    if MacOS.xcode_version < "4.2"
+      puts <<-EOS.undent
+        We couldn't detect gcc 4.2.x. Some formulae require this compiler.
 
-    EOS
+      EOS
+    end
   elsif gcc_42 < RECOMMENDED_GCC_42
     puts <<-EOS.undent
       Your gcc 4.2.x version is older than the recommended version. It may be advisable