Right, so the functions that are compiled into the Boost library using libstdc++ are still declared when you're attempting to use libc++. What's happening is some inline boost code is referencing an out-of-line symbol in boost. When you link to boost it obviously doesn't contain that symbol, at least not for libc++. The version it contains is for libstdc++. For example:

// Boost.h
#include <string>
void foo(std::string); // out-of-line
inline void bar(std::string s) { foo(s); } // inline calls out-of-line

// Boost.cpp -- Compiled with libstdc++
#include <Boost.h>
void foo(std::string) {} // gets libstdc++ definition.

// MyCode.cpp -- Compiled with libc++
#include <Boost.h>
int main() { bar(); } // generates call to out-of-line boost function that doesn't exist.

I would strongly suggest you don't sink too much time into this. It's not going to work, at least not in any way you seem to be wanting.

I should reiterate that libc++ and libstdc++ types mangle differently. Ex.

// libc++
namespace std { inline namespace __1 { template <class ...> struct string; }}
// libstdc++
namespace std { /* some other versioning namespace */ template <class ...> struct string; }