doctor: add check for osx-gcc-installer

This might not detect all cases, but should detect the two most
common ones.

Library/Homebrew/cmd/doctor.rb

@@ -232,6 +232,17 @@ def __check_clt_up_to_date
   end
 end
 
+def check_for_osx_gcc_installer
+  if (MacOS.version < 10.7 || MacOS::Xcode.version < "4.1") && \
+    MacOS.clang_version == "2.1" then <<-EOS.undent
+    You have osx-gcc-installer installed.
+    Homebrew doesn't support osx-gcc-installer, and it is known to cause
+    some builds to fail.
+    Please install Xcode #{MacOS::Xcode.latest_version}.
+    EOS
+  end
+end
+
 def check_for_stray_developer_directory
   # if the uninstaller script isn't there, it's a good guess neither are
   # any troublesome leftover Xcode files