Simplify the fix: found to way to override teh cache file.

Author: DanielCChen

llvm/CMakeLists.txt

@@ -1190,16 +1190,19 @@ endif()
 # Build with _XOPEN_SOURCE on AIX, as stray macros in _ALL_SOURCE mode tend to
 # break things. In this case we need to enable the large-file API as well.
 if (UNIX AND ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
-          add_compile_definitions(_XOPEN_SOURCE=700)
-          add_compile_definitions(_LARGE_FILE_API)
-          add_compile_options(-pthread)
+  add_compile_definitions(_XOPEN_SOURCE=700)
+  add_compile_definitions(_LARGE_FILE_API)
+  add_compile_options(-pthread)
 
   # Modules should be built with -shared -Wl,-G, so we can use runtime linking
   # with plugins.
   string(APPEND CMAKE_MODULE_LINKER_FLAGS " -shared -Wl,-G")
 
   # Also set the correct flags for building shared libraries.
   string(APPEND CMAKE_SHARED_LINKER_FLAGS " -shared")
+
+  # Set LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF as AIX doesn't support it
+  set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR OFF CACHE BOOL "" FORCE)
 endif()
 
 # Build with _XOPEN_SOURCE on z/OS.