USES=compiler:c++11-lib on FreeBSD 8.4

[Geoff Speicher]

Not sure if ports is the right place for this post so apologies in advance
if I'm not in the right place.

I'm porting an application that requires c++11, and I'm trying to create
the port on a FreeBSD 8.4 box. From everything I have read, it appears that
c++11 is only supported in 9.1+ when world has been built with the c++11
toolchain. However, the various ports switches such as
USES=compiler:c++11-lib don't yield any warnings on 8.4, yet don't seem to
work either. Can anyone comment on whether or not this option is supposed
to work on 8.4?

Taking ports out of the equation for a moment, I can't compile the
following c++11 file:

// test.cpp
#include <type_traits>
#include <string>

int main() {
  int number = 10;
  std::string value;
  value = std::to_string(number);
}
// EOF

clang++33 fails on a new c++11 include file:

$ clang++33 -std=c++11 test.cpp
test.cpp:1:10: fatal error: 'type_traits' file not found
#include <type_traits>
         ^
1 error generated.


g++47 at least finds the include file but doesn't appear to define
std::to_string as it should:

$ g++47 -std=c++11 test.cpp
test.cpp: In function 'int main()':
test.cpp:7:11: error: 'to_string' is not a member of 'std'

I also tried installing devel/libc++ and that fails trying to include
xlocale.h, which only appears to exist in 9.1 and later. Am I missing
something here or is c++11 simply not supported on 8.4? Assuming the
latter, is there a way to automatically make ports fail with a meaningful
error message on <9.0 when attempting to use c++11-lang or c++11-lib?

Geoff