CLANG as a preprocessor.

I need some help with Clang. I’m working in a project where I need to use CLANG as a preprocessor. After building Clang following the instructions from the site:
I have successfully manage to compile very simple programs. However when I tried to compile another that needs to include a library like for instance:
#include “mpi.h” // openmpi library

I get an error:
fatal error: ‘mpi.h’ file not found

Searching in the net for possible solutions I found the tutorial:

In which it is explained that with the aid of the helper class called ‘InitHeaderSearch’. However now I get the following error:

fatal error: ‘clang/Driver/InitHeaderSearch.h’ file not found

After doing a find on the ‘InitHeaderSearch*’ I don’t find it neither, instead what I found was the InitHeaderSearch.cpp
Any Ideas on how to proceed?
Thanks for your help,