- The phrase `#ifdef CAP_` should never appear anywhere.
- The phrase `#ifndef CAP_` should appear only in sysconfig.h.
- The phrases `#if CAP_` and `#if !CAP_` may appear wherever,
as long as "sysconfig.h" is included first.
The rules for `CAP_FOO` equally apply to `ISFOO`.
There are many one-off macros still tested with `#ifdef`,
including `HAVE_ACHIEVEMENTS`, `PRINT_ACHIEVEMENTS`,
`FAKEWEB`, `FAKE_SDL`, `EASY`, and `WHATEVER`. I don't
have much grasp on what these are used for or how they're
configured, so I'm leaving them alone.
Here we're casting a function pointer to `void*`, which is not doable
with our implicit-conversions-only `hr::voidp(x)` helper, and is
not even doable with a `static_cast`; we must `reinterpret_cast`.
