Allow eternal to be used as a subdirectory

[Qix-]

Kind of strange it hasn't been caught before, but using CMAKE_SOURCE_DIR always refers to the root-most CMake project, not the Eternal directory (wherever it might exist).

Changing it to CMAKE_CURRENT_SOURCE_DIR allows Eternal to properly set up the include paths of the interface target to refer to the subdirectory's location instead of the project root (which makes no sense).