New in version 3.17.
Specifies a semicolon-separated list of configurations to build for a target in build.ninja if no :<Config> suffix is specified in the Ninja Multi-Config generator. If it is set to all, all configurations from CMAKE_CROSS_CONFIGS are used. If it is not specified, it defaults to CMAKE_DEFAULT_BUILD_TYPE.
For example, if you set CMAKE_DEFAULT_BUILD_TYPE to Release, but set CMAKE_DEFAULT_CONFIGS to Debug or all, all <target> aliases in build.ninja will resolve to <target>:Debug or <target>:all, but custom commands will still use the Release configuration.
The value of this variable must be a subset of CMAKE_CROSS_CONFIGS or be the same as CMAKE_DEFAULT_BUILD_TYPE. It must not be specified if CMAKE_DEFAULT_BUILD_TYPE or CMAKE_CROSS_CONFIGS is not used.
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https://cmake.org/cmake/help/v3.19/variable/CMAKE_DEFAULT_CONFIGS.html