fdopendir visibility in dirent.h

[Niclas Rosenvik <nros%netbsd.org@localhost>]

When fixing packages for pkgsrc-2021Q4
I found that fdopendir is only visible
when _NETBSD_SOURCE is defined.
The thing is that it is a function
defined in POSIX.1 2008 edition.
So the function declaration
is not visible when programs set
-D_POSIX_C_SOURCE=200809L or
-D_XOPEN_SOURCE=700 .
I plan on putting

#if (_POSIX_C_SOURCE - 0) >= 200809L || (_XOPEN_SOURCE - 0) >= 700 || \
    defined(_NETBSD_SOURCE)
#ifndef __LIBC12_SOURCE__
DIR *fdopendir(int);
#endif
#endif

in dirent.h so that it is also
visible to POSIX and XOPEN users 
if it is okay and no one
objects to it.

Niclas Rosenvik