diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -1,5 +1,6 @@
 # See https://libcxx.llvm.org/docs/BuildingLibcxx.html for instructions on how
 # to build libcxx with CMake.
+# Dummy change.
 
 if (NOT IS_DIRECTORY "${CMAKE_CURRENT_LIST_DIR}/../libcxxabi")
   message(FATAL_ERROR "libc++ now requires being built in a monorepo layout with libcxxabi available")
diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -87,12 +87,19 @@
 
 include(CheckLibraryExists)
 include(CheckCCompilerFlag)
-
-# Disable use of the installed C++ standard library when building runtimes. If
-# MSVC is true, we must be using the clang-cl driver, which doesn't understand
-# these flags.
-if (NOT MSVC)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++ -nostdlib++")
+include(CheckCXXCompilerFlag)
+
+# Disable use of the installed C++ standard library when building runtimes.
+# Check for -nostdlib++ first; if there's no C++ standard library yet,
+# all check_cxx_compiler_flag commands will fail until we add -nostdlib++
+# (or -nodefaultlibs).
+check_c_compiler_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
+if (LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
+endif()
+check_cxx_compiler_flag(-nostdinc++ LLVM_RUNTIMES_SUPPORT_NOSTDINCXX_FLAG)
+if (LLVM_RUNTIMES_SUPPORT_NOSTDINCXX_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++")
 endif()
 
 # Avoid checking whether the compiler is working.