build: specify a compiler target for pythonkit on macOS

Because the rest of the components have a deployment target of 10.13 explicitly, pass the same triple for the PythonKit build.

Author: compnerd

utils/swift_build_support/swift_build_support/products/pythonkit.py

@@ -56,6 +56,9 @@ def build(self, host_target):
                     self.args.install_destdir),
                 '-D', 'CMAKE_MAKE_PROGRAM={}'.format(self.toolchain.ninja),
                 '-D', 'CMAKE_Swift_COMPILER={}'.format(swiftc),
+                # SWIFT_ENABLE_TENSORFLOW
+                '-DCMAKE_Swift_COMPILER_TARGET=x86_64-apple-macosx10.13' if host_target.startswith('macosx') else '',
+                # SWIFT_ENABLE_TENSORFLOW END
                 '-B', self.build_dir,
                 '-S', self.source_dir,
             ])