So close...

the problem we're running into is that "-isysroot" is only added for universal builds, not for regular builds. Furthermore unixccompiler doesn't know that is should always look in the SDK and not in the regular location (for system locations). 

I guess we should switch to:
- Never add -isysroot to CFLAGS (change to configure)
- Teach _osx_support about locating the default sdk root, both for Xcode and "command line tools" (_osx_support.default_sdk_root())
- Use that new function in setup.py and unixccompiler.

The first item might affect older systems where Xcode shipped with 2 SDKs (current and previous OS version). 

Is that something we can back port to 3.8 and 3.9? 

A less clean, but smaller, change is to teach the configure script to always add -isysroot on macOS, not just when doing a universal build.