CMake: add an option that forces the use of internal SDK

rdar://22866512

Swift SVN r32316

CMakeLists.txt

@@ -126,6 +126,10 @@ option(SWIFT_EMBED_BITCODE_SECTION
     "If non-empty, embeds LLVM bitcode binary sections in the standard library and overlay binaries for supported platforms"
     FALSE)
 
+option(SWIFT_DARWIN_USE_INTERNAL_SDK
+    "If true, selects the internal SDK on Darwin"
+    FALSE)
+
 option(SWIFT_RUNTIME_CRASH_REPORTER_CLIENT
     "Whether to enable CrashReporter integration"
     FALSE)
@@ -231,11 +235,10 @@ find_program(LIPO "lipo")
 
 if("${SWIFT_NATIVE_LLVM_TOOLS_PATH}" STREQUAL "")
   set(SWIFT_CROSS_COMPILING FALSE)
-  set(SWIFT_USE_INTERNAL_SDK FALSE)
 else()
   set(SWIFT_CROSS_COMPILING TRUE)
   # Cross compiling tools requires the internal SDK.
-  set(SWIFT_USE_INTERNAL_SDK TRUE)
+  set(SWIFT_DARWIN_USE_INTERNAL_SDK TRUE)
 endif()
 
 # Reset CMAKE_SYSTEM_PROCESSOR if not cross-compiling.

cmake/modules/SwiftConfigureSDK.cmake

@@ -83,12 +83,15 @@ macro(configure_sdk_darwin
   set(SWIFT_SDK_${prefix}_PATH "" CACHE PATH "Path to the ${name} SDK")
 
   if(NOT SWIFT_SDK_${prefix}_PATH)
-    if(${SWIFT_USE_INTERNAL_SDK})
+    if(${SWIFT_DARWIN_USE_INTERNAL_SDK})
       # Prefer the internal SDK, if present, for building C code and linking.
       execute_process(
           COMMAND "xcrun" "--sdk" "${xcrun_name}.internal" "--show-sdk-path"
           OUTPUT_VARIABLE SWIFT_SDK_${prefix}_PATH
           OUTPUT_STRIP_TRAILING_WHITESPACE)
+      if(NOT EXISTS "${SWIFT_SDK_${prefix}_PATH}/System/Library/Frameworks/module.map")
+        message(FATAL_ERROR "Internal ${name} SDK not found at SWIFT_SDK_${prefix}_PATH.")
+      endif()
     endif()
   endif()