Log Message:
Only configure libuuid using ksh on NetBSD 6.

The build of ksh is all but broken on modern systems. PR pkg/52877
introduced ksh as CONFIGURE_SHELL to fix NetBSD 6, but this causes issues
on NetBSD-8 and later (PR pkg/53686). Arguably, the workaround could be
dropped, since we stopped supporting NetBSD 6.

Fix proposed by David H. Gutteridge, thanks!