doctor: be more forgiving about GREP_OPTIONS

Currently, `brew doctor` complains about GREP_OPTIONS being set,
regardless of its value. The cmake formula, for which this check was
introduced, is more specific about what actually makes it fail.

GREP_OPTIONS='--color=auto' will not cause the build to fail, so don't
warn about this.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/cmd/doctor.rb

@@ -684,10 +684,10 @@ end
 
 def check_for_GREP_OPTIONS
   target_var = ENV['GREP_OPTIONS'].to_s
-  unless target_var.empty?
+  unless target_var.empty? or target_var == '--color=auto'
     puts <<-EOS.undent
     $GREP_OPTIONS was set to \"#{target_var}\".
-    Having $GREP_OPTIONS set can cause CMake builds to fail.
+    Having $GREP_OPTIONS set this way can cause CMake builds to fail.
 
     EOS
   end