#ifdef THREAD removed from mysql.cc.
               No reason was found for this limitation to persist.

per-file comments:
  client/mysql.cc
Bug#54466      client 5.5 built from source lacks "pager" support
        now we have USE_POPEN always if not __WIN__
  mysql-test/r/mysql.result
Bug#54466      client 5.5 built from source lacks "pager" support
        result updated.
  mysql-test/t/mysql.test
Bug#54466      client 5.5 built from source lacks "pager" support
        test case added.