New in version 3.17.
Specifies the configuration to use by default in a build.ninja file in the Ninja Multi-Config generator. If this variable is specified, build.ninja uses build rules from build-<Config>.ninja by default. All custom commands are executed with this configuration. If the variable is not specified, the first item from CMAKE_CONFIGURATION_TYPES is used instead.
The value of this variable must be one of the items from CMAKE_CONFIGURATION_TYPES.
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https://cmake.org/cmake/help/v3.19/variable/CMAKE_DEFAULT_BUILD_TYPE.html