nbdkit has strict ABI guarantees: we don't break old plugins written
in C and distributed as binaries.  But to actually test that we need
to run old plugins against the current nbdkit.  This directory
contains old binary plugins and is used to test that we don't break
the ABI unintentionally.

The binary plugins were prepared by compiling old versions of nbdkit
and copying plugins/file/.libs/nbdkit-file-plugin.so.  Therefore the
source of these files is simply the older versions of nbdkit itself.

If there is a problem with using/distributing binaries with your free
software distribution then you can simply do:

  rm -rf tests/old-plugins/

and the ABI tests will be skipped.

The old plugins are of course architecture specific.  They will only
be tested if there is a matching

  `uname -m`/`uname -s`

directory here.

- RWMJ December 2019
