To build parrot with clang, A C compiler targeting LLVM, you first need a bleeding edge version of clang.

You should build it from svn following [http://clang.llvm.org/get_started.html these instructions].

Then configure and build parrot with

{{{
perl Configure.pl --cc=/path/to/your/llvm/tree/Debug/bin/clang --without-gmp
make
}}}