[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:

> Would you like to remove `PURE_WINDOWS`, too?

Possibly, but not in this PR.

> we shouldn't provide them and instead check CMAKE_SYSTEM_NAME directly

Please note that I do not insist on this change, it is just a suggestion that I think makes sense.


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