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This adds yaml-cpp-config.cmake, yaml-cpp-config-version.cmake, and
yaml-cpp-targets.cmake to the cmake install. As a result, cmake's
find_package can easily find yaml-cpp for software that depends on
yaml-cpp.

Add code to install cmake files to $CMAKE_INSTALL_PREFIX/CMake on
Windows, which is the de-facto standard.

Closes jbeder/yaml-cpp#336 jbeder/yaml-cpp#127