These are present in the Autotools build system as a separate library, libsfiof,
linked into libsfio. They are also present in the MS Build build system inlined
into libsfio itself. This change copies the MS Build approach.
This did not cause problems because these sources only exist as fallback
implementations for functions that are also implemented as macros. All current
usages see the macro definitions so never call these functions.
Fixing this will allow removing the macro versions in an upcoming change.