Do not build the separate Multi-threaded (MT) library libyazthread.la.

Do not build the separate Multi-threaded (MT) library libyazthread.la.
Instead make the library libyaz.la multi-threaded. Threading as a whole
can still be disabled by configure option --disable-threads. Previosly,
with older GLIBCs there were problems with MT libs under Apache. This
has now been fixed a long time ago.