ENH: allow to overwrite macOS platform tag via _PYTHON_HOST_PLATFORM

Use sysconfig.get_platform() instead of platform.mac_ver() to base the
platform tag computation.  The ability to overwrite the value returned
by former via the _PYTHON_HOST_PLATFORM environment variable is used
in macOS extension modules cross-compilation.

See mesonbuild#222 for background.

mesonpy/_tags.py

@@ -84,7 +84,19 @@ def get_abi_tag() -> str:
 
 
 def _get_macosx_platform_tag() -> str:
-    ver, x, arch = platform.mac_ver()
+    ver, _, arch = platform.mac_ver()
+
+    # Override the architecture with the one provided in the
+    # _PYTHON_HOST_PLATFORM environment variable.  This environment
+    # variable affects the sysconfig.get_platform() return value and
+    # is used to cross-compile python extensions on macOS for a
+    # different architecture.  We base the platform tag computation on
+    # platform.mac_ver() but respect the content of the environment
+    # variable.
+    try:
+        arch = os.environ.get('_PYTHON_HOST_PLATFORM', '').split('-')[2]
+    except IndexError:
+        pass
 
     # Override the macOS version if one is provided via the
     # MACOS_DEPLOYMENT_TARGET environment variable.

tests/test_tags.py

@@ -43,6 +43,14 @@ def test_macos_platform_tag(monkeypatch):
             assert next(packaging.tags.mac_platforms((major, minor))) == mesonpy._tags.get_platform_tag()
 
 
+@pytest.mark.skipif(platform.system() != 'Darwin', reason='macOS specific test')
+def test_python_host_platform(monkeypatch):
+    monkeypatch.setenv('_PYTHON_HOST_PLATFORM', 'macosx-12.0-arm64')
+    assert mesonpy._tags.get_platform_tag().endswith('arm64')
+    monkeypatch.setenv('_PYTHON_HOST_PLATFORM', 'macosx-11.1-x86_64')
+    assert mesonpy._tags.get_platform_tag().endswith('x86_64')
+
+
 def wheel_builder_test_factory(monkeypatch, content):
     files = defaultdict(list)
     files.update({key: [(pathlib.Path(x), os.path.join('build', x)) for x in value] for key, value in content.items()})