This can be seen when the `./configure` step fails to detect `HAVE_SHM_*`,
e.g. due to missing a necessary dependency to compile the test scripts.
(Run `./configure`, run `yum install libtool-ltdl-devel` for missing dependencies,
then run `make`, and php can end up built with 0 shared memory opcache caches)
Give a clearer error message than `unknown`
Searching for `opcache "Fatal Error Unable to allocate shared memory segment of"
"unknown: No such file or directory"` reveals issues such as
https://github.com/termux/termux-packages/issues/2234
Closes GH-5615
f3efb9e3fb