[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:

> Looking at the `PURE_WINDOWS` above, `confix-ix.cmake` seems the right place to provide such variables.

My point is that we shouldn't provide them and instead check CMAKE_SYSTEM_NAME directly, as I believe is done in most places.

https://github.com/llvm/llvm-project/blob/a9604cd3212beb6cdb41a79f248723140ab9939d/llvm/cmake/modules/AddLLVM.cmake#L336).

These variables only save a couple of lines in the first `if` below.


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