GCC fails to compile stdlib stuff for C++ 11

out

mark@thukydides ~$ c++ -O2 -Wall -std=c++11 -lpthread -c test.cpp
test.cpp: In function ‘int main()’:
test.cpp:10:3: error: ‘mutex’ is not a member of ‘std’
   std::mutex foo;
   ^

test.cpp

#include <thread>
#include <mutex>

int main()
{
  int n[] = {4,7,6,1,2};
  int r;
  auto f = [&](int j) { r = j; };
  
  std::mutex foo;
  for (auto i : n)
    f(i);
  return 0;
}