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.