_GLIBCXX_USE_CXX11_ABI disabled on RHEL6 and RHEL7?

I have gcc 5.2.1 on RHEL6 and RHEL7, and it looks like _GLIBCXX_USE_CXX11_ABI gets disabled. It's not working even if I manually run -D_GLIBCXX_USE_CXX11_ABI=1 -std=c++14. This means I won't get small string optimization feature. For example, the output of following code always have 8 and 'micro not set'. For SSO, size of std::string should be at least 16 if we look at code bits/basic_string.h. Any workaround?

#include <string>
#include <iostream>

int main()
{
    std::cout << sizeof(std::string) << std::endl;

#if _GLIBCXX_USE_CXX11_ABI
    std::cout << "macro set" << std::endl;
#else
    std::cout << "macro not set" << std::endl;
#endif

}

Solution

bugzilla.redhat has below reply

Jakub Jelinek 2018-02-19 06:08:00 EST

We've tried hard, but it is not possible to support this, neither on RHEL6 nor on RHEL7, which is why it is forcefully disabled. It will work in RHEL8 (and be the default there as well).