this is ugly and stupid, but now that the *64 symbol names exist, a
lot of broken GNU software detects them in configure, then either
breaks during build due to missing off64_t definition, or attempts to
compile without function declarations/prototypes. "fixing" it here is
easier than telling everyone to add yet another feature test macro to
their builds.
this is a case of poorly written man pages not matching the actual
implementation, and why i hate implementing nonstandard interfaces
with no actual documentation of how they're intended to work.
