Added null-impl for apiv2 to fix loading on CLISP with no threads support

[svetlyak40wt]

Probably this will fix loading on other systems too, when there is not
threading support.

In old BT version implementation files were included condiitionally
if there is implementation name and :thread-support in features. But
when apiv2 was introduced, it dropped check for thread support.

This pull returns this check back.

[svetlyak40wt]

Pull failed on CI.

SBCL, SBCL-32, ECL and CLISP can't be installed

The problem is the same:

Installing SBCL...
Downloading https://cdn.cddr.org/ci/sbcl-2.0.7+r00-x86-64-linux-binary.tar.bz2...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (60) SSL certificate problem: unable to get local issuer certificate
More details here: https://curl.haxx.se/docs/sslcerts.html
curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.
Failed to download https://cdn.cddr.org/ci/sbcl-2.0.7+r00-x86-64-linux-binary.tar.bz2.

I've checked https://cdn.cddr.org/ and browser considers it valid

[sionescu]

I've fixed the CI, but I don't intend to support unthreaded implementations for APIv2. I could change my mind, but at the moment it seems unlikely.

[svetlyak40wt]

The reason why I want this patch to be accepted is that some systems can depend on bordeaux-threads, like it Rove does. It is only changes *default-special-bindings*:

https://github.com/fukamachi/rove/blob/bbc7b8e3e300266b44780fc926aee2962e649392/reporter.lisp#L55-L60

But because of this dependency, I'm unable to use Rove for running tests on implementations compiled without threads support.

When I'm loading Rove in my tests, quicklisp loads bordeaux-threads as a dependency and fails with this error:

An unhandled error condition has been signalled: READ from #<INPUT BUFFERED FILE-STREAM CHARACTER
     #P"/Users/art/projects/lisp/cl-info/.qlot/dists/ultralisp/software/sionescu-bordeaux-threads-20200801171406/apiv2/impl-clisp.lisp"
@10>: there is no package with name "MT"

If you don't want to support unthreaded implementation, then at least we needed to make bordeaux-threads compile on these implementations without error and without adding :thread-support to the *features*.

This way we will have a chance to use :thread-support feature for conditional compilation inside Rove.

[sionescu]

What other non-threaded Lisp implementations are you using, besides Clisp ? I seriously suggest dropping Clisp support.

[svetlyak40wt]

I want some of my tools to work for a large number of implementations. Is this bad?

[sionescu]

No, but I think it's time to drop support for unthreaded implementations.