msetup: make xcode unsupported warning actually show

Until we invoke interpreter.Interpreter(b, ...) the coredata options
still have their default values and thus cannot be used sensibly.
Currently the warning never shows (other than, unsurprising in
retrospect, during --internal regenerate).

mesonbuild/msetup.py

@@ -177,8 +177,6 @@ class MesonApp:
         mlog.initialize(env.get_log_dir(), self.options.fatal_warnings)
         if self.options.profile:
             mlog.set_timestamp_start(time.monotonic())
-        if env.coredata.options[mesonlib.OptionKey('backend')].value == 'xcode':
-            mlog.warning('xcode backend is currently unmaintained, patches welcome')
         with mesonlib.BuildDirLock(self.build_dir):
             self._generate(env)
 
@@ -264,6 +262,11 @@ class MesonApp:
 
             # Post-conf scripts must be run after writing coredata or else introspection fails.
             intr.backend.run_postconf_scripts()
+
+            # collect warnings about unsupported build configurations; must be done after full arg processing
+            # by Interpreter() init, but this is most visible at the end
+            if env.coredata.options[mesonlib.OptionKey('backend')].value == 'xcode':
+                mlog.warning('xcode backend is currently unmaintained, patches welcome')
         except Exception as e:
             mintro.write_meson_info_file(b, [e])
             if 'cdf' in locals():