New in version 3.17.
Specifies a semicolon-separated list of configurations available from all build-<Config>.ninja files in the Ninja Multi-Config generator. This variable activates cross-config mode. Targets from each config specified in this variable can be built from any build-<Config>.ninja file. Custom commands will use the configuration native to build-<Config>.ninja. If it is set to all, all configurations from CMAKE_CONFIGURATION_TYPES are cross-configs. If it is not specified, or empty, each build-<Config>.ninja file will only contain build rules for its own configuration.
The value of this variable must be a subset of CMAKE_CONFIGURATION_TYPES.
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https://cmake.org/cmake/help/v3.19/variable/CMAKE_CROSS_CONFIGS.html