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

[Sergei Barannikov 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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s-barannikov wrote:

I will humbly suggest to not define any of these variables in global scope and inline the checks.
They look like they can conflict with CMake-predefined variables and / or variables defined in other places.
(note that this file is `include`d, which means the names are not local to this file).


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