fix posix.mq_receive compilation with cpp backend (#20710)

* fix posix.mq_receive compilation with cpp backend

* only enable runnableExample on posix

* linux, not posix

* simplify example

* add test

* mqueue.h does not exist on MacOS

* place test case in own block

* drop runnableExamples

tests/stdlib/tposix.nim

@@ -7,7 +7,7 @@ outputsub: ""
 when not defined(windows):
 
   import posix
-  import std/syncio
+  import std/[assertions, syncio]
 
   var
     u: Utsname
@@ -18,3 +18,46 @@ when not defined(windows):
   writeLine(stdout, u.nodename)
   writeLine(stdout, u.release)
   writeLine(stdout, u.machine)
+
+  when not (defined(nintendoswitch) or defined(macos) or defined(macosx)):
+    block:
+      type Message = object
+        value: int
+
+      const MQ_PATH: cstring = "/top_level_file"
+      const MQ_PRIORITY: cuint = 170
+      const MQ_MESSAGE_SIZE: csize_t = csize_t(sizeof(Message))
+
+      let mqd_a: posix.MqAttr = MqAttr(mq_maxmsg: 10, mq_msgsize: clong(MQ_MESSAGE_SIZE))
+      let writable: posix.Mqd = posix.mq_open(
+        MQ_PATH,
+        posix.O_CREAT or posix.O_WRONLY or posix.O_NONBLOCK,
+        posix.S_IRWXU,
+        addr(mqd_a)
+      )
+      let readable: posix.Mqd = posix.mq_open(
+        MQ_PATH,
+        posix.O_RDONLY or posix.O_NONBLOCK,
+        posix.S_IRWXU,
+        addr(mqd_a)
+      )
+
+      let sent: Message = Message(value: 88)
+      block:
+        let success: int = writable.mq_send(
+          cast[cstring](sent.addr),
+          MQ_MESSAGE_SIZE,
+          MQ_PRIORITY
+        )
+        doAssert success == 0, $success
+
+      block:
+        var buffer: Message
+        var priority: cuint
+        let bytesRead: int = readable.mq_receive(
+          cast[cstring](buffer.addr),
+          MQ_MESSAGE_SIZE,
+          priority
+        )
+        doAssert buffer == sent
+        doAssert bytesRead == int(MQ_MESSAGE_SIZE)