CMAKE_CURRENT_SOURCE_DIR

The path to the source directory currently being processed.

This the full path to the source directory that is currently being processed by cmake.

When run in -P script mode, CMake sets the variables CMAKE_BINARY_DIR, CMAKE_SOURCE_DIR, CMAKE_CURRENT_BINARY_DIR and CMAKE_CURRENT_SOURCE_DIR to the current working directory.

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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.5/variable/CMAKE_CURRENT_SOURCE_DIR.html