[llvm] [cmake] Hardcode some `check_include_file` checks (PR #104706)

[Vlad Serebrennikov via llvm-commits]

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@@ -3,6 +3,44 @@ if( WIN32 AND NOT CYGWIN )
   set(PURE_WINDOWS 1)
 endif()
 
+if (CMAKE_SYSTEM_NAME STREQUAL "AIX")
+  set(AIX 1)
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Endilll wrote:

Would you like to remove `PURE_WINDOWS`, too?

https://github.com/llvm/llvm-project/pull/104706