libyaz.la (whole source) is compiled with threads

libyaz.la (whole source) is compiled with threads

If thread support is enabled enabled, all source of YAZ is compiled
with treading support. This allows core utilities such as yaz_log to
use thread facilities. Previously (YAZ 3 and YAZ 4 releases), only
libyaz_server.la was using thread utilities. This commit moves mutex
utilities (mutex.c) from libyaz_server.la to libyaz.la.