[cmake] Add an option to prefer public SDK in find_darwin_sdk_dir

[kubamracek]

Summary

Adds a CMake option DARWIN_PREFER_PUBLIC_SDK, off by default. When on, this prefers to use the public SDK, even when an internal one is present. With this, it's easy to emulate a build that the public buildbots are doing.

[george.karpenkov]

LGTM

compiler-rt/trunk/cmake/Modules/CompilerRTDarwinUtils.cmake

 include(CMakeParseArguments)
 
 # On OS X SDKs can be installed anywhere on the base system and xcode-select can
 # set the default Xcode to use. This function finds the SDKs that are present in
 # the current Xcode.
 function(find_darwin_sdk_dir var sdk_name)
+  set(DARWIN_PREFER_PUBLIC_SDK OFF CACHE BOOL "Prefer Darwin public SDK, even when an internal SDK is present.")
+  if(NOT DARWIN_PREFER_PUBLIC_SDK)
     # Let's first try the internal SDK, otherwise use the public SDK.
     execute_process(
       COMMAND xcodebuild -version -sdk ${sdk_name}.internal Path
       RESULT_VARIABLE result_process
       OUTPUT_VARIABLE var_internal
       OUTPUT_STRIP_TRAILING_WHITESPACE
       ERROR_FILE /dev/null
     )
+  endif()
   if((NOT result_process EQUAL 0) OR "" STREQUAL "${var_internal}")
     execute_process(
       COMMAND xcodebuild -version -sdk ${sdk_name} Path
       RESULT_VARIABLE result_process
       OUTPUT_VARIABLE var_internal
       OUTPUT_STRIP_TRAILING_WHITESPACE
       ERROR_FILE /dev/null
     )