std::shared_ptr is a C++11 feature, whose location (and existence) in STL
implementations of previous C++ standards varied -- one widespread example
is GCC's libstdc++, which has a shared_ptr implementation in <tr1/memory>
that was shipped long before C++11.

However, including it unconditionally breaks the build if any other STL
implementation (such as libc++) is used instead.

We now check if std::shared_ptr is present in the <memory> header and then
try std::tr1::shared_ptr as a fallback.

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