Testing the behaviour may be challenging, however, because I suspect the CentOS 7 implementation of posix_spawn() is just racy, and they simply documented that they don't care.
Thinking about it more, I'm afraid it's not a testable behaviour. Not only isn't there any way to force the race since it entirely happens inside a libc function, but also, the test would require running code on the build machine, which doesn't work for cross-builds and people would have to manually set the sysdep anyway. It seems like --with-sysdep-posixspawn=no, as you did, is the easiest workaround. -- Laurent
