Clang rejects a declared (but never defined and never used) `static`
function. Change the declaration to be the value of a macro that will
be expanded only when the function is both defined and used.
Reported-by: kreatordxx <https://github.com/dxx-rebirth/dxx-rebirth/issues/295>
Fixes: 55822d0b4d ("Add wrappers for more precise Valgrind physfs diagnostics")