The #4486 fix for `CACHELINE_SIZE` probe on qemu-user is incomplete, libzmq v4.3.5 compilation still fails

There is an already merged past PR trying to fix a compilation error on s390x when running in qemu-user emulation.

* https://github.com/zeromq/libzmq/pull/4486

## Steps to reproduce

I am still seeing this issue happening for myself:

(https://github.com/opendatahub-io/notebooks/actions/runs/21263557165/job/61196980881#step:19:2288)

```
      /tmp/tmp2d4gyw1v/build/_deps/bundled_libzmq-src/src/curve_client.cpp:3:
      /tmp/tmp2d4gyw1v/build/_deps/bundled_libzmq-build/platform.hpp:22:28:
      error: ‘undefined’ was not declared in this scope
         22 | #define ZMQ_CACHELINE_SIZE undefined
            |                            ^~~~~~~~~
      /tmp/tmp2d4gyw1v/build/_deps/bundled_libzmq-src/src/command.hpp:191:26:
      note: in expansion of macro ‘ZMQ_CACHELINE_SIZE’
        191 | __attribute__ ((aligned (ZMQ_CACHELINE_SIZE)))
            |                          ^~~~~~~~~~~~~~~~~~
```

This is PyZMQ building the bundled libzmq.

## Investigation

```
cmake -P /dev/stdin <<'EOF'
set(CACHELINE_SIZE "undefined")
if(CACHELINE_SIZE STREQUAL ""
   OR CACHELINE_SIZE EQUAL 0
   OR CACHELINE_SIZE EQUAL -1
   OR CACHELINE_SIZE EQUAL "undefined")
  set(ZMQ_CACHELINE_SIZE 64)
else()
  set(ZMQ_CACHELINE_SIZE ${CACHELINE_SIZE})
endif()

message(STATUS "ZMQ_CACHELINE_SIZE: ${ZMQ_CACHELINE_SIZE}")
EOF
```

produces

    -- ZMQ_CACHELINE_SIZE: undefined

## Suggested fix

Use [`STREQUAL`](https://cmake.org/cmake/help/latest/command/if.html#strequal) instead of [`EQUAL`](https://cmake.org/cmake/help/latest/command/if.html#equal) to compare with the `"undefined"` string.

Perform the STREQUAL exact-match string comparisons first and the coercing EQUAL comparisons only after that.

```
cmake -P /dev/stdin <<'EOF'
set(CACHELINE_SIZE "undefined")
if(CACHELINE_SIZE STREQUAL ""
   OR CACHELINE_SIZE STREQUAL "undefined"
   OR CACHELINE_SIZE EQUAL 0
   OR CACHELINE_SIZE EQUAL -1)
  set(ZMQ_CACHELINE_SIZE 64)
else()
  set(ZMQ_CACHELINE_SIZE ${CACHELINE_SIZE})
endif()

message(STATUS "ZMQ_CACHELINE_SIZE: ${ZMQ_CACHELINE_SIZE}")
EOF
```

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