[SystemZ][z/OS] Define _XOPEN_SOURCE=600 for dlopen (llvm#127254)

On z/OS, dlopen is guarded by _XOPEN_SOURCE=600 so define it when
checking for the symbol.

Author: abhina-sree

llvm/cmake/config-ix.cmake

@@ -390,7 +390,14 @@ if (NOT PURE_WINDOWS)
   if( HAVE_LIBDL )
     list(APPEND CMAKE_REQUIRED_LIBRARIES dl)
   endif()
+  # Add the _XOPEN_SOURCE macro on z/OS, as certain test(s) use dlopen
+  if (ZOS)
+    list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_XOPEN_SOURCE=600")
+  endif()
   check_symbol_exists(dlopen dlfcn.h HAVE_DLOPEN)
+  if (ZOS)
+    list(REMOVE_ITEM CMAKE_REQUIRED_DEFINITIONS "-D_XOPEN_SOURCE=600")
+  endif()
   if( HAVE_LIBDL )
     list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES dl)
   endif()