catdoc: fails with clang

This software uses undefined behavior (decrementing a null pointer)
that compiles one way on GCC and another way on Clang with optimizations.

Formula/catdoc.rb

@@ -5,6 +5,10 @@ class Catdoc < Formula
   url 'http://ftp.wagner.pp.ru/pub/catdoc/catdoc-0.94.2.tar.gz'
   sha1 '50ce9d7cb24ad6b10a856c9c24183e2b0a11ca04'
 
+  fails_with :clang do
+    cause "The source uses undocumented behavior (decrementing null)."
+  end
+
   def install
     # catdoc configure says it respects --mandir=, but does not.
     ENV['man1dir'] = man1