diff --git a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
--- a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
+++ b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
@@ -52,6 +52,12 @@
     endif()
   endforeach()
 
+  # If CMAKE_SYSTEM_NAME is not set explicitly in the arguments passed to us,
+  # reflect CMake's own default.
+  if (NOT _cmake_system_name)
+    set(_cmake_system_name "${CMAKE_HOST_SYSTEM_NAME}")
+  endif()
+
   if(NOT ARG_TOOLCHAIN_TOOLS)
     set(ARG_TOOLCHAIN_TOOLS clang lld llvm-ar llvm-ranlib llvm-nm llvm-objdump)
     if(_cmake_system_name STREQUAL Darwin)