For projects where the CMakeLists.txt is not in the root of the project tree this command can be used to specify the sub-folder the CMake project file is in.

The value is used to set the ExternalProject_Add command sub-option SOURCE_SUBDIR.

It is meant to be used in the project definition at project/<ProjectName>/hunter.cmake:

    # SOURCE_SUBDIR will be forwarded to ExternalProject_Add command