atscppapi/shared_ptr.h includes ink_autoconf.h. ink_autoconf.h is not installed (and must not be installed). This makes it impossible for people to consume the C++ API.
It is totally reasonable IMHO to expect to be able to build plugins with a different toolchain than Traffic Server was built with. Consider installing devtoolset-3 and building plugins against a distribution package, or building plugins with clang.