Since Clang doesn't keep backwards binary compatibility, it's necessary
to rebuild when Clang (major version?) changes. This was broken
because e.g. check.cxx didn't include plugin.hxx, and so it didn't depend
on config_clang.h . Now simply force timestamp change if config_clang.h
changes.
This still needs re-running configure though.

Change-Id: Icbc404b37105599f1ca6c8996f5a3d45d50082db
Reviewed-on: https://gerrit.libreoffice.org/72976
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>