BUG: 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

@@ -83,8 +83,8 @@ def get_abi_tag() -> str:
     return abi.replace('.', '_').replace('-', '_')
 
 
-def _get_macosx_platform_tag() -> str:
-    ver, x, arch = platform.mac_ver()
+def _get_macosx_platform_tag(platform) -> str:
+    ver, arch = platform.split('-', 1)
 
     # Override the macOS version if one is provided via the
     # MACOS_DEPLOYMENT_TARGET environment variable.
@@ -133,7 +133,7 @@ def _get_macosx_platform_tag() -> str:
 def get_platform_tag() -> str:
     platform = sysconfig.get_platform()
     if platform.startswith('macosx'):
-        return _get_macosx_platform_tag()
+        return _get_macosx_platform_tag(platform)
     if _32_BIT_INTERPRETER:
         # 32-bit Python running on a 64-bit kernel.
         if platform == 'linux-x86_64':