Sample Project can't reference llvm headers

The sample project (project/sample) builds fine out of the tree until you try to use a llvm header. What is the correct way to get this sample project access to the llvm headers?

/* LLVM Header File*/
#include “llvm/Support/DataTypes.h”

llvm[2]: Compiling sample.c for Release+Asserts build
sample.c:14:10: fatal error: ‘llvm/Support/DataTypes.h’ file not found
#include “llvm/Support/DataTypes.h”
^
1 error generated.