Hangs during operator tuning & how to disable it

[matteosal]

We have experienced problems with the mxnet library hanging at loading time on some Linux machines, specifically during the operator tuning phase (mxnet::op::OperatorTune). I don't have precise information about where the hang happened exactly and the machine details, but on a CentOS system we could avoid the hang by setting the environment variable MXNET_USE_OPERATOR_TUNING=0 before loading, while on an ARM machine we also had to rebuild the library with the cmake option USE_OPERATOR_TUNING=OFF.

So what's the relationship between the env variable and the build option?