[krbdev.mit.edu #5560] threads on Solaris

[Ken Raeburn via RT]

The k5-thread.h header has some comments about problems with figuring
out if the thread support was linked in.  I think that was for Solaris
9 or some earlier version, but the comment doesn't say.

In Solaris 10, the thread support is in libc, with the symbols weakly
defined.  (And libpthread is just a filter object that re-exports
those symbols, non-weakly I think, from libc.  It has no code in it at
all.)  So we should not be using weak references and tests to see if
the functions are there; instead we should just directly use the
native thread support.

Ken