[Linter] Detect warning disabling compiler flags.

lib/cocoapods/command/linter.rb

@@ -226,6 +226,7 @@ module Pod
         messages << "The summary should end with a dot"                     if spec.summary !~ /.*\./
         messages << "The description should end with a dot"                 if spec.description !~ /.*\./ && spec.description != spec.summary
         messages << "Comments must be deleted"                              if text.scan(/^\s*#/).length > 24
+        messages << "Warnings must not be disabled (`-Wno' compiler flags)" if spec.compiler_flags.split(' ').any? {|flag| flag.start_with?('-Wno') }
 
         if (git_source = source[:git])
           messages << "Git sources should specify either a tag or a commit" unless source[:commit] || source[:tag]