Samuel, the program complaining about not being able to find the C Preprocessor is the system "dtrace" executable. Looks like a dependency there.

To check this hypotesis, create a file called "a.d", with the following content:

"""
provider test {
  probe abc(int);
};

"""

and try to generate the header file with the following command:

"""
dtrace -C -h -s a.d
"""

If that command complains about being unable to find the C Preprocessor, well, you need a C Preprocessor.

If dtrace doesn't obey environment variables pointing to the preprocessor, that would be a dtrace bug. You could create a symbolic link to "clang" executable, but that would be an ugly hack.