diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -462,7 +462,7 @@
           -DLLVM_TARGETS_TO_BUILD="host" \
           -DRUNTIMES_BUILD_ALLOW_DARWIN=ON \
           -DLLVM_ENABLE_ASSERTIONS=ON \
-          -DRUNTIMES_LIBUNWIND_TEST_CONFIG="llvm-libunwind-shared.cfg.in"
+          -DLIBUNWIND_TEST_CONFIG="llvm-libunwind-shared.cfg.in"
 
     echo "+++ Running the libc++ and libc++abi tests"
     ${NINJA} -C "${BUILD_DIR}" check-runtimes
diff --git a/llvm/runtimes/CMakeLists.txt b/llvm/runtimes/CMakeLists.txt
--- a/llvm/runtimes/CMakeLists.txt
+++ b/llvm/runtimes/CMakeLists.txt
@@ -340,6 +340,17 @@
     endforeach()
   endif()
 
+  # If we're passed any option like <project>_FOO, pass it through to the sub-builds.
+  foreach(runtime ${LLVM_ENABLE_RUNTIMES})
+    string(TOUPPER "${runtime}" runtime_upper)
+    foreach(variableName ${variableNames})
+      if (variableName MATCHES "^${runtime_upper}_.+")
+        string(REPLACE ";" "|" escaped_value "${${variableName}}")
+        list(APPEND ${name}_extra_args "-D${variableName}=${escaped_value}")
+      endif()
+    endforeach()
+  endforeach()
+
   if(NOT RUNTIMES_${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})