It appears support for the wstring data type is missing, at least in 
-current.
I've read on gcc mailing lists that the libstdc++ configure script
is rather picky about which platforms it enables wstring support on.
wstring is afaik part of ISO C++.

I used the following steps to get wstring support:

src/gnu/lib/libstdc++-v3_4$ echo "#define _GLIBCXX_USE_WCHAR_T 1" > 
arch/x86_64/c++config.h
src/gnu/lib/libstdc++-v3_4$ USETOOLS=never make
src/gnu/lib/libstdc++-v3_4$ su
# USETOOLS=never make install
# USETOOLS=never make includes

Those steps at least make the minimal test case work:

#include <string>
using namespace std;
wstring foo;

Any chance to get this enabled? Should I send a PR?

-Tobias