[llvm] [CMake] Passthrough variables for packages to subbuilds (PR #107611)

[Shoaib Meenai via llvm-commits]

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@@ -156,6 +156,32 @@ function(llvm_ExternalProject_Add name source_dir)
     set_target_properties(${name}-clear PROPERTIES FOLDER "${ARG_FOLDER}")
   endif ()
 
+  set(DEFAULT_PASSTHROUGH_VARIABLES
+    LibEdit_INCLUDE_DIRS
+    LibEdit_LIBRARIES
+    ZLIB_INCLUDE_DIR
+    ZLIB_LIBRARY
+    zstd_INCLUDE_DIR
+    zstd_LIBRARY
+    LIBXML2_LIBRARY
+    LIBXML2_INCLUDE_DIR
+    CURL_INCLUDE_DIR
+    CURL_LIBRARY
+    HTTPLIB_INCLUDE_DIR
+    HTTPLIB_HEADER_PATH
+    Python3_EXECUTABLE
+    Python3_LIBRARIES
+    Python3_INCLUDE_DIRS
+    Python3_RPATH
+    )
+  foreach(variable ${DEFAULT_PASSTHROUGH_VARIABLES})
+    get_property(is_value_set CACHE ${variable} PROPERTY VALUE SET)
+    if(${is_value_set})
+      get_property(value CACHE ${variable} PROPERTY VALUE)
+      list(APPEND CMAKE_CACHE_DEFAULT_ARGS "-D${variable}:STRING=${value}")
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smeenai wrote:

Do we not need to replace `;` with `|` to have list variables be passed through correctly? The documentation isn't clear on whether `LIST_SEPARATOR` applies to `CMAKE_CACHE_DEFAULT_ARGS`.

https://github.com/llvm/llvm-project/pull/107611