How to write Clang plugin Makefile with binary download LLVM and Clang?

Hi! I’am new to LLVM and Clang. I just followed this tutorial and tried to build a Clang plugin on my PC.
The tutorial’s makefile is simple like this:

CLANG_LEVEL := …/…
LIBRARYNAME = PluginExample

SOURCES := PluginExample.cpp

LINK_LIBS_IN_SHARED = 0
SHARED_LIBRARY = 1

include $(CLANG_LEVEL)/Makefile

However, this tutorial’s LLVM and Clang is built by compiling the source, while I get LLVM(and Clang) binary online directly(with apt-get install on Ubuntu). It’s installed in /usr/lib. So, I’m confused where my CLANG_LEVEL is, what does it exactly mean?