Fixes #4203: Move fix for intermittent mpirun crashes to v1.7

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r30507 [[BR]]
Stop listening thread and wait for it to terminate.
Fixes #4202: Move Sparc ASM message fix to v1.7

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r30503 [[BR]]
Clean up error message about Sparc v8/v8+, since the README now has better
information and the cases are getting too long for an AC_MSG_WARN.