it's really not that difficult to test for & include the sys/sysmacros.h header
directly when you're using the non-standard functions they provide.  this is
the API under BSD systems, and where Linux C libs have already moved/are moving
towards.  *this* is the deprecation period.

upstream glibc will take a more measured approach including updating their
docs, but realistically, no one is going to read those until they see breakage.
 the Gentoo changes are only in an opt-in dev-only build, so it too isn't
hitting people randomly.

as the upstream thread shows, if you used the (long time documented)
AC_HEADER_MAJOR macro in autoconf, then this wouldn't have impacted drm.