cannot compile <future> with gcc 4.6

["James K. Lowden" <jklowden%schemamania.org@localhost>]

On NetBSD 5.0.2 I cannot instantiate the new C++11 "future" class.  The
header file <future> guards the definition with _GLIBCXX_HAS_GTHREADS,
and that symbol is not defined.  

$ cat -n t.C &&  make t.o
     1  #include <future>
     2  
     3  future<int> foo;
c++  -O2 -g -std=c++0x -o t.o  -c t.C
t.C:3:1: error: 'future' does not name a type
*** Error code 1

$ which c++
/usr/pkg/gcc46/bin/c++

Is GNU's libc required to compile this program?  

--jkl