diff --git a/llvm/runtimes/CMakeLists.txt b/llvm/runtimes/CMakeLists.txt
--- a/llvm/runtimes/CMakeLists.txt
+++ b/llvm/runtimes/CMakeLists.txt
@@ -343,6 +343,19 @@
     endforeach()
   endif()
 
+  # If we're passed any option like RUNTIMES_<project>_FOO, pass it through as <project>_FOO.
+  # For example, RUNTIMES_LIBCXX_TEST_CONFIG will be passed down as LIBCXX_TEST_CONFIG.
+  foreach(runtime ${LLVM_ENABLE_RUNTIMES})
+    string(TOUPPER "${runtime}" runtime_upper)
+    foreach(variableName ${variableNames})
+      if (variableName MATCHES "RUNTIMES_${runtime_upper}_.+")
+        string(REPLACE "RUNTIMES_" "" new_name "${variableName}")
+        string(REPLACE ";" "|" new_value "${${variableName}}")
+        list(APPEND ${name}_extra_args "-D${new_name}=${new_value}")
+      endif()
+    endforeach()
+  endforeach()
+
   if(NOT RUNTIMES_${name}_LLVM_ENABLE_RUNTIMES AND NOT RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES)
     string(REPLACE ";" "|" LLVM_ENABLE_RUNTIMES_PASSTHROUGH "${LLVM_ENABLE_RUNTIMES}")
     list(APPEND ${name}_extra_args -DLLVM_ENABLE_RUNTIMES=${LLVM_ENABLE_RUNTIMES_PASSTHROUGH})