Trick is that a single header of no-op macros (~600 lines of C, plus as many lines of comments) is committed into your project. Then if you want those macros to actually validate things at compile-time, you do a build with some extra C++ files that #undef's and re-#define's those macros with industrial-strength C++ metaprogramming.

(It's basically high powered static analysis using the compiler you already have on your machine.)