dnl Available from the GNU Autoconf Macro Archive at:
dnl http://www.gnu.org/software/ac-archive/htmldoc/ac_cxx_have_ext_hash_set.html
dnl
AC_DEFUN([AC_CXX_HAVE_EXT_HASH_SET],
[AC_CACHE_CHECK(whether the compiler has ext/hash_set,
ac_cv_cxx_have_ext_hash_set,
[AC_REQUIRE([AC_CXX_NAMESPACES])
  AC_LANG_SAVE
  AC_LANG_CPLUSPLUS
  AC_TRY_COMPILE([#include <ext/hash_set>
#ifdef HAVE_NAMESPACES
using namespace std;
#endif],[hash_set<int> t; return 0;],
  ac_cv_cxx_have_ext_hash_set=yes, ac_cv_cxx_have_ext_hash_set=no)
  AC_LANG_RESTORE
])
if test "$ac_cv_cxx_have_ext_hash_set" = yes; then
   AC_DEFINE(HAVE_EXT_HASH_SET,,[define if the compiler has ext/hash_set])
fi
])