(not for landing) gn build: use xcrun on mac to find sdk path

[thakis]

Summary

This makes both Xcode and no-Xcode-but-commandline-tools-only builds Just Work, and it's The Right Thing To Do, but it makes gn take twice as long (35ms->77ms) and almost everyone has Xcode installed.

So don't land this, at least for now.

llvm/utils/gn/.gn

 # Since this can't be at the toplevel, you either need to pass
 # `--dotfile=llvm/utils/gn/.gn --root=.` to the `gn gen` command
 # or use llvm/utils/gn/gn.py which calls gn with these two flags added.
 
 buildconfig = "//llvm/utils/gn/build/BUILDCONFIG.gn"
 
 # Disallow all calls to exec_script. We should be very conservative about
 # whitelisting things here.
-exec_script_whitelist = []
+exec_script_whitelist = [
+  "//llvm/utils/gn/build/mac_sdk.gni",
+]
 
 # The normal GN setup is to have BUILD.gn files in-tree and a root BUILD.gn
 # file.  Since LLVM's GN build is unofficial, set secondary_source to tell GN
 # that e.g. llvm/lib/Demangle/BUILD.gn is found at
 # llvm/utils/gn/secondary/llvm/lib/Demangle/BUILD.gn and that the root BUILD.gn
 # file is at llvm/utils/gn/secondary/BUILD.gn
 secondary_source = "//llvm/utils/gn/secondary/"

llvm/utils/gn/build/mac_sdk.gni

 # Location of the mac sdk.
-# If that's not fixed, might want to shell out to xcrun at gn time to
-# retrieve this, but for now this seems to do the trick.
-mac_sdk_path = "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk"
+mac_sdk_path = exec_script("mac_sdk.py", [], "string")

llvm/utils/gn/build/mac_sdk.py

+#!/usr/bin/env python
+
+from __future__ import print_function
+
+import subprocess
+
+print(subprocess.check_output(['xcrun', '--show-sdk-path']).rstrip(), end='')