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* Export YAML::detail::node::m_amount

The internal header node/detail/node.h is included by public headers;
YAML::detail::node is implemented in the header itself, and thus it gets
inlined... except for its static m_amount class member, which is
instantiated in the library only. Right now all the symbols of yaml-cpp
are exported (nothing is hidden), so the linker will find node::m_amount
in the yaml-cpp library.

As solution/workaround, explicitly export YAML::detail::node::m_amount.

* CMake: use GenerateExportHeader

Make use of the GenerateExportHeader CMake module to generate the dll.h
header with export macros.

While the produced dll.h is different, the result should be the same,
i.e. nothing changes for yaml-cpp or its users.

* CMake: hide all the symbols by default

Hide all the symbols that are not explicitly exported with YAML_CPP_API.
This way the ABI will be way smaller, and only actually exposing the
public classes/functions.