diff --git a/clang/CMakeLists.txt b/clang/CMakeLists.txt
--- a/clang/CMakeLists.txt
+++ b/clang/CMakeLists.txt
@@ -758,6 +758,19 @@
     set(LTO_RANLIB)
   endif()
 
+  # Populate the passthrough variables
+  foreach(variableName ${CLANG_BOOTSTRAP_PASSTHROUGH} ${_BOOTSTRAP_DEFAULT_PASSTHROUGH})
+    if(DEFINED ${variableName})
+      if("${${variableName}}" STREQUAL "")
+        set(value "")
+      else()
+        string(REPLACE ";" "|" value "${${variableName}}")
+      endif()
+      list(APPEND PASSTHROUGH_VARIABLES
+        -D${variableName}=${value})
+    endif()
+  endforeach()
+
   # Find all variables that start with BOOTSTRAP_ and populate a variable with
   # them.
   get_cmake_property(variableNames VARIABLES)
@@ -774,19 +787,6 @@
     endif()
   endforeach()
 
-  # Populate the passthrough variables
-  foreach(variableName ${CLANG_BOOTSTRAP_PASSTHROUGH} ${_BOOTSTRAP_DEFAULT_PASSTHROUGH})
-    if(DEFINED ${variableName})
-      if("${${variableName}}" STREQUAL "")
-        set(value "")
-      else()
-        string(REPLACE ";" "|" value "${${variableName}}")
-      endif()
-      list(APPEND PASSTHROUGH_VARIABLES
-        -D${variableName}=${value})
-    endif()
-  endforeach()
-
   ExternalProject_Add(${NEXT_CLANG_STAGE}
     DEPENDS clang-bootstrap-deps
     PREFIX ${NEXT_CLANG_STAGE}