You need to build llvm/libclang (https://clang.llvm.org/docs/Tooling.html) or install libclang.
Set LIBCLANG_HOME env. var to be the directory where you've installed libclang.

Command line to jextract clang's Index.h

$ sh ./compile.sh

Command line to parse and print AST of a sample .c file

$ sh ./run.sh hello.c