diff --git a/CMakeLists.txt b/CMakeLists.txt
index 31638c287..efcb4fe9c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -64,6 +64,7 @@ set(COLLECTIONS_SINGLE_MODULE YES)
 set(COLLECTIONS_FOUNDATION_TOOLCHAIN_MODULE YES)
 
 set(SwiftFoundation_MACRO "" CACHE STRING "Path to Foundation macro plugin")
+set(USE_PREBUILT_SDK NO CACHE BOOL "A full prebuilt SDK is available, so use its headers")
 
 # Make sure our dependencies exists
 include(FetchContent)
diff --git a/Sources/_FoundationCShims/CMakeLists.txt b/Sources/_FoundationCShims/CMakeLists.txt
index 1798a5bb0..341056f1b 100644
--- a/Sources/_FoundationCShims/CMakeLists.txt
+++ b/Sources/_FoundationCShims/CMakeLists.txt
@@ -17,10 +17,13 @@ add_library(_FoundationCShims STATIC
     string_shims.c
     uuid.c)
 
-target_include_directories(_FoundationCShims PUBLIC include)
 
-target_compile_options(_FoundationCShims INTERFACE
-  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xcc -fmodule-map-file=${CMAKE_CURRENT_SOURCE_DIR}/include/module.modulemap>")
+if(NOT USE_PREBUILT_SDK)
+  target_include_directories(_FoundationCShims PUBLIC include)
+
+  target_compile_options(_FoundationCShims INTERFACE
+    "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xcc -fmodule-map-file=${CMAKE_CURRENT_SOURCE_DIR}/include/module.modulemap>")
+endif()
 
 set_property(GLOBAL APPEND PROPERTY SWIFT_FOUNDATION_EXPORTS _FoundationCShims)