I wrote:
> It seems like the more productive approach would be to try to identify
> the right sysroot to use. I wonder if there is some less messy way
> to find out the compiler's default sysroot than to scrape it out of
> -v output.
This is, of course, not terribly well documented by Apple. But
Mr. Google suggests that "xcrun --show-sdk-path" might serve.
What does that print for you?
regards, tom lane