We *always* generate this file, and we depend on its existence.

The idea behind HAVE_CONFIG_H was being able to include random files
from different projects, back in a time where "libraries" were literally
just random files instead of actual shared objects.

Since we're not in the '80s any more, and our build system(s) define
HAVE_CONFIG_H *and* generate the config.h header file, we don't need a
conditional guard around its inclusion.