ac_cxx_namespaces

If the compiler can prevent names clashes using namespaces, define HAVE_NAMESPACES.

AC_DEFUN([AC_CXX_NAMESPACES],
[AC_CACHE_CHECK(whether the compiler implements namespaces,
ac_cv_cxx_namespaces,
[AC_LANG_SAVE
 AC_LANG_CPLUSPLUS
 AC_TRY_COMPILE([namespace Outer { namespace Inner { int i = 0; }}],
                [using namespace Outer::Inner; return i;],
 ac_cv_cxx_namespaces=yes, ac_cv_cxx_namespaces=no)
 AC_LANG_RESTORE
])
if test "$ac_cv_cxx_namespaces" = yes; then
  AC_DEFINE(HAVE_NAMESPACES,,[define if the compiler implements namespaces])
fi
])