llvm/utils/gn/build/mac_sdk.gni

declare_args() {
  # Set to true if you don't have Xcode installed, but do have the commandline
  # tools.
  mac_use_commandline_tools_sdk = false
}

# Location of the mac sdk.
# The correct way to do this is to call xcrun (https://reviews.llvm.org/D70835),
# but that makes `gn gen` take twice as long and almost everyone has Xcode
# installed.  So require that people who don't have it installed set a gn arg.
if (mac_use_commandline_tools_sdk) {
  mac_sdk_path = "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk"
} else {
  mac_sdk_path = "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk"
}