autoconf issue using -stdc=gnu++0x option in AC_CHECK_HEADER macro.

[Benjamin Kosnik]

> I have a special question about the AC_HEADER macro in autoconf. I'm
> trying to text for
> the existence of the unordered_set header file in C++.

I wrote up autoconf macros for this here:
http://gcc.gnu.org/onlinedocs/libstdc++/17_intro/backwards_compatibility.html

See "Support for C++TR1 dialect"

I'm waiting on feedback and then hope to merge these into the autoconf
macro repository.

> Unfortunately,
> I will got a message,
> that I have to use the -std=gnu++0x if I want to include this header
> file.

Yes, as <unordered_set> is a C++0x standard header.

If you'd like to use C++98, I suggest using TR1's <tr1/unordered_set>

best,
benjamin