Custom GCC 4.8.2 on RHEL6 gives build error with std::shared_ptr

I am using CMake 2.8.11 and GCC 4.8.2. I was building some C++ code which used std::shared_ptr which built fine in MS VS 2012 but when I tried the same on RHEL6 using GCC 4.8.2, I promptly ran into the following error:

error: 'shared_ptr' is not a member of 'std'

I found this question with responses that I thought addressed and I promptly added -std=c++11 to my CMAKE_CXX_FLAGS, but I still keep running into the error. I add the flag in CMake simply using:

SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11" CACHE STRING "Add C++ 11 flags")

I set my custom compiler in CMake using:

SET(GCC_DIR "</path/to/custom>/gcc")
SET(CMAKE_CXX_COMPILER "${GCC_DIR}/bin/g++ CACHE FILEPATH "CXX compiler")
SET(CMAKE_C_COMPILER "${GCC_DIR}/bin/gcc CACHE FILEPATH "C compiler")

The include is

#include <memory>

which in turn has

#include <bits/shared_ptr.h>

which defines the shared_ptr class. So I'm not sure why I keep getting the error (and yes I cleared cached and rebuilt after adding the -std=c++11 compiler option). Any ideas are very much appreciated.

EDIT 1:

I created a simple program (main.cpp) as follows:

#include <memory>
#include <iostream>

int main() {
  std::shared_ptr<int> pint = std::make_shared<int>();
  std::cout << "Pint points to " << pint.get() << "\n";
  return 0;
}

Then I built it using <path/to/custom/>g++ main.cpp -o prog and promptly ran into the same error (above). Next I did: <path/to/custom/>g++ -std=c++11 main.cpp -o prog and it compiles & runs OK. For my real application, I added the -std=c++11 flag to linker flags as well (in addition to compiler flags) in my CMake config system, but I still see the same error. Proceeding to check the CMakeCache to see if the flags are property registered, but any ideas are appreciated.

EDIT 2:

Suprisingly, I found in CMakeCache that the -std=c++11 flag is not being added to the CMAKE_CXX_FLAGS, etc. So this must have to do with the error. I am trying to fix it so that it actually takes this flag. Thanks all.

Solution

The answer confirms the hunch in EDIT 2 of my question. Apparently CMake 2.8.x is not appending to the variable CMAKE_CXX_FLAGS using the SET command using the syntax shown in my question (as per the documentation); I tried other variants of the SET command to append, to no avail.

So finally, instead of appending, I assigned separately for the case when C++11 is to be enabled and when it is to be disabled, as follows:

IF(USE_C++11)
...
ELSE(USE_C++11)
...
ENDIF(USE_C++11)

This worked fine. Thanks to @nos for the idea to make an isolated example.