diff --git a/CMakeLists.txt b/CMakeLists.txt
index 939a29e48..8a96ef6cd 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -15,6 +15,14 @@ cmake_minimum_required(VERSION 3.16)
 project(SwiftCollections
   LANGUAGES C Swift)
 
+if(NOT SWIFT_SYSTEM_NAME)
+  if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
+    set(SWIFT_SYSTEM_NAME macosx)
+  else()
+    set(SWIFT_SYSTEM_NAME "$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>")
+  endif()
+endif()
+
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules)
 
 set(CMAKE_Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift)
diff --git a/cmake/modules/SwiftSupport.cmake b/cmake/modules/SwiftSupport.cmake
index 0ce99fb82..8631a4ccf 100644
--- a/cmake/modules/SwiftSupport.cmake
+++ b/cmake/modules/SwiftSupport.cmake
@@ -59,12 +59,7 @@ endfunction()
 # Sets ${result_var_name} with the converted OS name derived from
 # CMAKE_SYSTEM_NAME.
 function(get_swift_host_os result_var_name)
-  if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
-    set(${result_var_name} macosx PARENT_SCOPE)
-  else()
-    string(TOLOWER ${CMAKE_SYSTEM_NAME} cmake_system_name_lc)
-    set(${result_var_name} ${cmake_system_name_lc} PARENT_SCOPE)
-  endif()
+  set(${result_var_name} ${SWIFT_SYSTEM_NAME} PARENT_SCOPE)
 endfunction()
 
 function(_install_target module)