Commit 380fe7d4548a99bfcfc1594b6f0b3dd2369978f1 broke the functionality on
OpenBSD as it has no sys/sysmacros.h. So replicate commit
08572e7db2120bc45db732d02409dfd3346b8e51 but use explicit OS checks instead of
AC_HEADER_MAJOR.

Signed-off-by: Ingo Feinerer <feinerer@logic.at>