Mac SDK include path apocalypse

[jjwilke]

Newer versions of xcode will no longer have a /usr/include in the path, meaning many macport/homebrew/default compilers will NOT bring basic library headers into the include path by default. The MAC SDK must be explicitly specified with -isysroot

We have a couple ways to fix this. One is to give an explicit

--with-sdk=PATH_TO_SDK

option and force the user to specify it. Otherwise, we can try to detect OS and version and find ourselves using

xcrun --sdk macosx --show-sdk-path

[berquist]

This doesn't seem to be an issue anymore.

[berquist]

Nope, I was wrong. I am finding that, on macOS 15.5 (and possibly a little earlier on the 15.x line) with Apple Clang 17, with Command Line Tools installed but not XCode, the C++ compiler doesn't include the STL properly.