compile Hello LLVM Pass in the source tree

I have seen this webpage :

But this webpage may be a little older. There is Makefile in
lib/Transforms/Hello. The content is a little different from the website.
And I am confused about the sentence - "First, configure and build LLVM.
This needs to be done directly inside the LLVM source tree rather than
in a separate objects directory."! I get from the wiki : LLVM should be
installed outside the LLVM source tree. (in-tree build is not supported)