[llvm] [z/OS] Improve compiler options on z/OS (PR #166415)

[Abhina Sree via llvm-commits]

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@@ -1264,10 +1264,10 @@ endif()
 # Build with _XOPEN_SOURCE on z/OS.
 if (CMAKE_SYSTEM_NAME MATCHES "OS390")
   add_compile_definitions(_XOPEN_SOURCE=600)
+  add_compile_definitions(_XPLATFORM_SOURCE)
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abhina-sree wrote:

nit: can we add a comment that we need this for O_CLOEXEC

https://github.com/llvm/llvm-project/pull/166415