RT3984: Fix clang compiler warning on Mac OS X where %ld is used for uint64_t.

clang suggests %llu instead, but it isn't clear that is portable on
all platforms.

C99 and above define a handy macro for us, so we try to use that
definition and fall back to current definition if needed (though we
switch to 'u' for unsigned).

Reviewed-by: Matt Caswell <matt@openssl.org>