Bazel Builds Fail on macOS 10.15

[Steven Smith]

I’m trying to build a bunch of tensorflow and related ports whose upstreams use the wonderful bazel build system.

These used to work a few months ago, but now are all throwing the error below.

Does anyone know how to resolve this issue.

E.g. see:
https://github.com/macports/macports-ports/pull/10294 <https://github.com/macports/macports-ports/pull/10294>
https://github.com/macports/macports-ports/pull/10295 <https://github.com/macports/macports-ports/pull/10295>
…

sudo port -dv test py39-dm-tree

> ERROR: /opt/local/var/macports/build/_opt_local_ports_python_py-dm-tree/py39-dm-tree/work/d8451668cbe25ab61b4db805f3a8f026/external/com_google_absl/absl/base/BUILD.bazel:119:11: C++ compilation of rule '@com_google_absl//absl/base:base' failed: I/O exception during sandboxed execution: xcrun failed with code 1.
> This most likely indicates that SDK version [10.15] for platform [MacOSX] is unsupported for the target version of xcode.
> Process exited with status 1
> stdout: stderr: xcodebuild: error: SDK "macosx10.15" cannot be located.
> xcodebuild: error: SDK "macosx10.15" cannot be located.
> xcrun: error: unable to lookup item 'Path' in SDK 'macosx10.15'
> Target //tree:_tree failed to build



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