[patch] fix thread test cases for solaris.

[Paolo Carlini]

.. was having another look to thread.cc - close to branching better
double checking our exports - and I'm under the impression we are likely
exporting too much: some of the thread functions apparently can be
inline, are very, very simple or involve just a single function call
(e.g., destructor, joinable, default constructor, swap). Unless you have
special reasons, please review that too...

Paolo.