new: Detect dirent.d_type support at configure time

Support for dirent.d_type is OS-specific.  Previously, we used
_DIRENT_HAVE_D_TYPE to detect support for this, but this is apparently
a glic-ism (FreeBSD, for example, supports d_type, but does not define
this).  Since there's no cross-platform way to detect support for
dirent.d_type, detect it using a test compile at configure time.