When building for newlib HAVE_OPENAT and HAVE_UNLINKAT are (sometimes?)
defined, but <fcntl.h> is only included when HAVE_DIRENT_H is defined.
Since directory iterators are completely useless without <dirent.h>,
just override the HAVE_OPENAT and HAVE_UNLINKAT detection when we don't
have <dirent.h>.
libstdc++-v3/ChangeLog:
* src/filesystem/dir-common.h (_GLIBCXX_HAVE_DIRFD): Undefine
when <dirent.h> is not available.
(_GLIBCXX_HAVE_UNLINKAT): Likewise.
inline DIR* opendir(const char*) { return nullptr; }
inline dirent* readdir(DIR*) { return nullptr; }
inline int closedir(DIR*) { return -1; }
+#undef _GLIBCXX_HAVE_DIRFD
+#undef _GLIBCXX_HAVE_UNLINKAT
#endif
} // namespace __gnu_posix