Revert my last change. It was stupid, which I if I had been thinking clearly

Revert my last change.  It was stupid, which I if I had been thinking clearly
I would have seen *before* I commited anything.  So, include isc/thread.h,
isc/mutex.h and isc/condition.h even if ISC_PLATFORM_USETHREADS is not defined.

(What caused me to bother with this at all was a problem that I resolved
a few days ago by fixing configure ... though now that I think about it,
that probably means there is some other latent problem with inconsistent
definitions that could maybe be handled better.  I'll look into it more
later, AFTER vacation.)