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This is what I've been able to gather:

There are two implementations of the standard C++ library available on OS X: libstdc++ and libc++. They are not binary compatible and libMLi3 requires libstdc++.

On 10.8 and earlier libstdc++ is chosen by default, on 10.9 libc++ is chosen by default. To ensure compatibility with libMLi3, we need to choose libstdc++ manually.

To do this, add -stdlib=libstdc++ to the linking command.

Note: the Makefile supplied with the MathLink examples uses the c++ command for linking (not cc or ld). In this case the -lstdc++ option can be removed from the linking command. However if you use cc for linking, then it's necessary to specify both of -stdlib=libstdc++ -lstdc++.