_SOURCE_DIR

From Get docs
Cmake/docs/3.21/variable/project-name source dir


_SOURCE_DIR

Top level source directory for the named project.

A variable is created with the name used in the project() command, and is the source directory for the project. This can be useful when add_subdirectory() is used to connect several projects.

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https://cmake.org/cmake/help/v3.21/variable/PROJECT-NAME_SOURCE_DIR.html