Don't treat clew as part of an "external" clcc module

There is no obvious authoritative upstream for clew anyway, so it causes
philosophical problems for distros. For a while, we used to use a zip archive
from the "clcc" project on SourceForge that included clew.c and
clew.h. (Before that we also just had clew.c and clew.h in our source repo.)

So, drop the external/clcc module and have clew.c and clew.h in the source
repo again. But this time clew is in a module of its own, not in sc.

This re-introduces "No need to have OpenCL optional at configure-time"

This reverts commit 764836cb00e8e6dfd2ab48e080a166ec90359e01.

Change-Id: I413142f4f9f8399489f9c3e5327132822f07a454
Reviewed-on: https://gerrit.libreoffice.org/13368
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>