support building two annoy indexes at the same time using separate threads
support building two annoy indexes at the same time using separate threads

tjrileywisc/annoy,pombredanne/annoy,spotify/annoy,pombredanne/annoy,eddelbuettel/annoy,tjrileywisc/annoy,spotify/annoy,pombredanne/annoy,eddelbuettel/annoy,tjrileywisc/annoy,spotify/annoy,spotify/annoy,pombredanne/annoy,eddelbuettel/annoy,tjrileywisc/annoy,eddelbuettel/annoy