meson: error out when swift unavailable or disabled and cplayer enabled

without swift mpv as a standalone app is unusable on macOS, since the
cocoa main main loop doesn't exists. furthermore most of the other
needed functionalities like video outputs, contexts, platform features,
etc are also missing.

without swift mpv is really only usable as libmpv.

meson.build

@@ -1540,6 +1540,10 @@ swift = get_option('swift-build').require(
 )
 features += {'swift': swift.allowed()}
 
+if features['cocoa'] and not features['swift'] and get_option('cplayer')
+    error('cplayer enabled but no suitable swift version could be found or was disabled!')
+endif
+
 swift_sources = []
 if features['cocoa'] and features['swift']
     swift_sources += files('osdep/mac/application.swift',