> What is the the return on investment for this feature in terms of performance?

I did not suggest a change for this issue to increase the build speed here. I do
not really know how often a different run time behaviour will be noticed on the
build systems.

I proposed the reuse of the AX_PTHREAD configuration script as a general
approach if you are interested to improve software portability around the POSIX
threads programming interface.

Regards,
Markus