meson: disable swift when cocoa not available or disabled

it doesn't make sense to build the swift parts without cocoa because
the swift bridge would be missing and it's impossible to interact
with the swift parts from obj-c, swift depends on cocoa/appkit/etc, and
the cocoa main function/main loop is missing making anything
cocoa/appkit/GUI crash on use.

like this it is possible to build mpv on macOS without cocoa, it will
fallback to the unix main function and mpv can run in 'headless' mode,
eg you can use it from CLI, it won't spawn any App icon/menu/etc, and
you can use everything that isn't related to cocoa/appkit/GUI like audio
playback. anything other will make mpv segfault, for example sdl2 and
its vo, since it tries to spawn a window.

meson.build

@@ -1526,7 +1526,11 @@ if macos_sdk_version != '0.0'
     add_project_link_arguments(objc_link_flags, language: ['c', 'objc'])
 endif
 
-xcrun = find_program('xcrun', required: get_option('swift-build').require(darwin))
+swift = get_option('swift-build').require(
+    darwin and features['cocoa'],
+    error_message: 'cocoa was not found',
+)
+xcrun = find_program('xcrun', required: swift)
 swift_ver = '0.0'
 if xcrun.found()
     swift_prog = find_program(run_command(xcrun, '-find', 'swiftc', check: true).stdout().strip())
@@ -1544,7 +1548,7 @@ sys.stdout.write(swift_ver)
 endif
 
 swift = get_option('swift-build').require(
-    darwin and macos_sdk_version.version_compare('>= 10.15') and swift_ver.version_compare('>= 4.1'),
+    swift.allowed() and macos_sdk_version.version_compare('>= 10.15') and swift_ver.version_compare('>= 4.1'),
     error_message: 'A suitable macos sdk version or swift version could not be found!',
 )
 features += {'swift': swift.allowed()}