How do I make an existing project (let’s say, MySQL) switch from generating a config.h that has #undef HAVE_FOO to one that generates #define HAVE_FOO 0 instead?
Why? Well, here’s why:
Testing for features should be done by including “config.h” and using #if HAVE_<feature>. Use of #if (rather than #ifdef) allows the gcc -Wundef flag to warn users about features which aren’t defined 1 or 0 in config.h.