b=1043075 revert to no hostbz2 when --with-system-bz2 r=glandium

--HG--
extra : rebase_source : 4d7c0bed8afe7c6edd5b39ca4f971667df35c139
b=1043772 assert ScopedXErrorHandler is used only on main thread r=nical

--HG--
extra : rebase_source : 737a78cabe0cb9a44786cd562cc1e34bafd7a2a0