Fix implementation of _tinydir_dirent_buf_size

[Cong]

The implementation of _tinydir_dirent_buf_size is lifted from an advisory on readdir_r, and uses lots of ifdefs to make it work across a large variety of platforms. The problem is that some of those ifdefs are using unimplemented preprocessor definitions like HAVE_FPATHCONF. These can work under an autoconf project but for tinydir, a different method must be used.

It may be possible to replace these with various feature test macros.