If the clang binary comes from a package which had been built before
any of our clang related sources were changed the last time, the timestamp
would be older and so there would be no rebuild. So do the stamp handling
the usual way, clang upgrades will work fine, downgrades will not, but
that's the same problem like with downgrading a library and its headers.

To somewhat mitigate the problem (Clang plugin doesn't get cleaned by
'make clean'), include the full Clang version (which includes SVN revision)
in config_clang.h and make all Clang plugin code include that, so
at least configure re-run will trigger a rebuild if necessary.

Change-Id: I993197f79e92e36105092c92c33b2e1db343e975