Esp. since LO links against a liborcus.a, this would otherwise cause linker
warnings like

  ld: warning: direct access in orcus::orcus_xml::read_file(char const*) to
  global weak symbol boost::unordered_detail::prime_list_template<unsigned
  long>::value means the weak symbol cannot be overridden at runtime. This was
  likely caused by different translation units being compiled with different
  visibility settings.

on Mac OS X (where HAVE_GCC_VISIBILITY_FEATURE is unset).

Change-Id: I6f09b61aa77b6d5c3f8b53407d8faff4fb3bb8a1