musl does not support legacy 32-bit-off_t whatsoever. off_t is always
64 bit, and correct programs that use off_t and the standard functions
will just work out of the box. (on glibc, they would require
-D_FILE_OFFSET_BITS=64 to work.) however, some programs instead define
_LARGEFILE64_SOURCE and use alternate versions of all the standard
types and functions with "64" appended to their names.

we do not want code to actually get linked against these functions
(it's ugly and inconsistent), so macros are used instead of prototypes
with weak aliases in the library itself. eventually the weak aliases
may be added at the library level for the sake of using code that was
originally built against glibc, but the macros will still be the
desired solution in the headers.

basically there are 3 choices for how to implement this variable-size
string member:
1. C99 flexible array member: breaks using dirent.h with pre-C99 compiler.
2. old way: length-1 string: generates array bounds warnings in caller.
3. new way: length-NAME_MAX string. no problems, simplifies all code.

of course the usable part in the pointer returned by readdir might be
shorter than NAME_MAX+1 bytes, but that is allowed by the standard and
doesn't hurt anything.

this is nonstandard but since POSIX reserved d_ prefix in dirent.h we
might as well define it unconditionally. some programs depend on it.