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
This still needs re-running configure though.
Reviewed-by: Luboš Luňák <firstname.lastname@example.org>