diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index c450ee5a3d72e..f192cd05b5a34 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -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) # Needed e.g. for O_CLOEXEC.
   add_compile_definitions(_OPEN_SYS) # Needed for process information.
   add_compile_definitions(_OPEN_SYS_FILE_EXT) # Needed for EBCDIC I/O.
   add_compile_definitions(_EXT) # Needed for file data.
-  add_compile_definitions(_UNIX03_THREADS) # Multithreading support.
   # Need to build LLVM as ASCII application.
   # This can't be a global setting because other projects may
   # need to be built in EBCDIC mode.