diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -77,12 +77,15 @@
 include(CheckLibraryExists)
 include(CheckCCompilerFlag)
 
-# We don't have libc++ (yet)...
-set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++ -nostdlib++")
-
-# ...but we need access to libc++ headers for CMake checks to succeed.
-if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR AND "libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -isystem ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR}/include")
+# Disable use of the installed C++ standard library when building runtimes.  If
+# the host compiler is MSVC or clang-cl, then the runtime compiler will be
+# clang-cl. clang-cl doesn't support -nostdinc++ or -isystem, so don't use them.
+if (NOT MSVC)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++ -nostdlib++")
+
+  if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR AND "libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -isystem ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR}/include")
+  endif()
 endif()
 
 # Avoid checking whether the compiler is working.