message: Doc: add nps dynamic module compatibility note
  Mention --with-cc-opt='-DNGX_HTTP_HEADERS' may be needed for binary compatibility of the dynamic module  with existing nginx installs provided by package managers.
diff: @@ -137,7 +137,9 @@ ngx_pagespeed module by adding this to the top of your main nginx configuration:
  If you're using dynamic modules to integrate with an already-built nginx, make
  sure you pass <code>./configure</code> the same arguments you gave it when
  building nginx the first time. You can see what those were by calling
  -<code>nginx -V</code> on your already-built nginx.
  +<code>nginx -V</code> on your already-built nginx. (Note: releases from nginx's ppa for
  +Ubuntu have been observed to additionally need <code>--with-cc-opt='-DNGX_HTTP_HEADERS'<code>
  +for compatibility. This will not be listed in the output of <code>nginx -V</code>.)
  </p>
  <p>
  