Mark Linux 2.4 as unsupported

Mark Linux 2.4 as unsupported

The Linux 2.4 series (and older) will not be supported platforms
for OpenAFS 1.8 and later.  Detect these systems at configure time
and direct users of those systems to the OpenAFS 1.6 series of releases.

These systems are believed to not be in common use with OpenAFS,
and retaining support for the LinuxThreads threading implementation
they require presents a maintenance burden that the project is
not equipped to deliver.  The project will be able to move forward
more quickly by desupporting these systems.

Code conditional on these old systems can be removed in subsequent
commits.

Change-Id: I679fc2390b35851f3b0457a846047c812bc03dba
Reviewed-on: http://gerrit.openafs.org/11799
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: Daria Brashear <shadow@your-file-system.com>