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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.