Drop hard Xcode dependency

XCODE_VERSION calls out to `xcodebuild`, which is only available
when having `Xcode` installed. The CommandLineTools are not
sufficient. To install Xcode, you must have an apple id to download
the Xcode.xip from apple.

We do not use xcodebuild anywhere in our build explicilty. At best
it appears to be a proxy for checking the linker or the compiler.
These should rather be done with
```
xcrun ld -version
```
or similar, and not by proxy through Xcode. The CLR should be
sufficient for building software on macOS.