- Path to ``{Clang,lld,Polly}``\'s source directory. Defaults to
- ``tools/{clang,lld,polly}``. ``{Clang,lld,Polly}`` will not be built when it
- is empty or it does not point to a valid path.
+ These variables specify the path to the source directory for the external
+ LLVM projects Clang, lld, and Polly, respectively, relative to the top-level
+ source directory. If the in-tree subdirectory for an external project
+ exists (e.g., llvm/tools/clang for Clang), then the corresponding variable
+ will not be used. If the variable for an external project does not point
+ to a valid path, then that project will not be built.