environment: handle all iOS variants as xnu

All of iOS, tvOS, visionOS, watchOS use the XNU kernel.  Report that and also
make them return true for is_darwin() which is really more like "is_xnu()".

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/markdown/Reference-tables.md

@@ -208,6 +208,8 @@ Meson natively.
 | ios-simulator       |                                 |
 | tvos                | Apple tvOS                      |
 | tvos-simulator      |                                 |
+| visionos            | Apple visionOS                  |
+| visionos-simulator  |                                 |
 | watchos             | Apple watchOS                   |
 | watchos-simulator   |                                 |
 

mesonbuild/envconfig.py

@@ -320,9 +320,9 @@ class MachineInfo(HoldableObject):
 
     def is_darwin(self) -> bool:
         """
-        Machine is Darwin (iOS/tvOS/OS X)?
+        Machine is Darwin (macOS/iOS/tvOS/visionOS/watchOS)?
         """
-        return self.system in {'darwin', 'ios', 'tvos'}
+        return self.system in {'darwin', 'ios', 'tvos', 'visionos', 'watchos'}
 
     def is_android(self) -> bool:
         """

mesonbuild/environment.py

@@ -487,6 +487,10 @@ KERNEL_MAPPINGS: T.Mapping[str, str] = {'freebsd': 'freebsd',
                                         'linux': 'linux',
                                         'cygwin': 'nt',
                                         'darwin': 'xnu',
+                                        'ios': 'xnu',
+                                        'tvos': 'xnu',
+                                        'visionos': 'xnu',
+                                        'watchos': 'xnu',
                                         'dragonfly': 'dragonfly',
                                         'haiku': 'haiku',
                                         'gnu': 'gnu',