Merge branch 'lindenlab/release' (6.4.19) into viewer/build

doc/contributions.txt

indra/edit-me-to-trigger-new-build.txt

indra/llappearance/llavatarjoint.cpp

indra/llcommon/llapr.h

indra/llcommon/llfasttimer.cpp

indra/llcommon/llfasttimer.h

indra/llcommon/llmemory.cpp

indra/llcommon/llmemory.h

indra/llcommon/llpreprocessor.h

indra/llcommon/llsdutil.cpp

indra/llcommon/llstring.h

indra/llcommon/llsys.cpp

indra/llcommon/llthread.cpp

indra/llcommon/lltimer.cpp

indra/llcommon/lltimer.h

indra/llcommon/lluuid.cpp

indra/llcommon/stdtypes.h

indra/llmath/llmath.h

indra/llmath/llsdutil_math.cpp

indra/llmessage/message.h