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This is in preparation for other patches that will make use of the
macro. The patch also removes #undef:ing the macro after its been
used to not make the header inclusion order critical. Otherwise,
the new header would always have to be the last of the yaml-cpp
headers to be included.

This way it's possible to build using older MSVC (<13) that don't
support this yet. Macro is undefined in each file where it is used so it
should stack well with other libs and sources.