Log Message:
python*: Avoid configure test for POSIX semaphores on Linux.

This attempts to build and run a program that uses POSIX semaphores.

This fails in a pbulk sandbox that doesn't contain /dev/shm, resulting
in a broken package where the idea that the platform doesn't support
POSIX semaphores is baked in forever. In newer Python versions,
this means Python doesn't even build properly.

XXX: We might want to avoid it on other platforms too...