feat: add the ability to declare safe tools in a cross-build environment. (#2317)

* Add the ability to declare safe tools in a cross-build environment.

* Add an xfail if cmake isn't available on the test machine.

* Placate linter regarding positional args.

* Rework test to provide more robust confirmation of safe tools.

* Remove a test skip condition that is no longer needed.

Co-authored-by: Joe Rickerby <joerick@mac.com>

* Rename the setting to xbuild-tools.

* Add docs to clarify that xbuild-tools is transitive.

* Raise a warning if xbuild-tools isn't defined.

* Correct a bad copy-paste in the schema generator.

* .. and now fix the indentation.

* Move sentinel handling earlier into the parsing process.

* Remove serialization from tests that won't start a test suite.

---------

Co-authored-by: Joe Rickerby <joerick@mac.com>

unit_test/options_test.py

@@ -618,3 +618,31 @@ def test_get_build_frontend_extra_flags_warning(
     )
     assert args == ["-Ca", "-Cb", "-1"]
     mock_warning.assert_called_once()
+
+
+@pytest.mark.parametrize(
+    ("definition", "expected"),
+    [
+        ("", None),
+        ("xbuild-tools = []", []),
+        ('xbuild-tools = ["cmake", "rustc"]', ["cmake", "rustc"]),
+    ],
+)
+def test_xbuild_tools_handling(tmp_path: Path, definition: str, expected: list[str] | None) -> None:
+    args = CommandLineArguments.defaults()
+    args.package_dir = tmp_path
+
+    pyproject_toml: Path = tmp_path / "pyproject.toml"
+    pyproject_toml.write_text(
+        textwrap.dedent(
+            f"""\
+            [tool.cibuildwheel]
+            {definition}
+            """
+        )
+    )
+
+    options = Options(platform="ios", command_line_arguments=args, env={})
+
+    local = options.build_options("cp313-ios_13_0_arm64_iphoneos")
+    assert local.xbuild_tools == expected