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This pull request adds two new exported variables to the CMake config:

YAML_CPP_LIBRARY_DIR - points to the directory containing the built library files
YAML_CPP_SHARED_LIBS_BUILT - boolean value, lets users know whether shared libraries were built or not

After configuring the file `yaml-cpp-config.cmake.in`, the result ends up with
empty variables.  (see also the discussion in #774).

Rework this file and the call to `configure_package_config_file` according the
cmake documentation
(https://cmake.org/cmake/help/v3.22/module/CMakePackageConfigHelpers.html?highlight=configure_package_config#command:configure_package_config_file)
to overcome this issue and allow a simple `find_package` after install.

As there was some discussion about the place where to install the
`yaml-cpp-config.cmake` file, e.g. #1055, factor out the install location into
an extra variable to make it easier changing this location in the future.

Also untabify CMakeLists.txt in some places to align with the other code parts in this file.

Add yaml-cpp-config.cmake and yaml-cpp-config-version.cmake files for importing yaml-cpp into external projects (through find_package).