Automatically Adding Preprocessor Defines to the Beginning of Files

What is the simplest way of modifying the clang lexer code to automatically
add a few preprocessor defines to the beginning of every file that is
complied using clang++? Thanks for any help!

What is the simplest way of modifying the clang lexer code to automatically
add a few preprocessor defines to the beginning of every file that is
complied using clang++? Thanks for any help

Why not just pass these as -D options on the command line?