message: Add xbox as a known supports expression term
diff: @@ -28,6 +28,7 @@ namespace vcpkg::PlatformExpression
  openbsd,
  osx,
  uwp,
  + xbox,
  android,
  emscripten,
  ios,
  @@ -54,6 +55,7 @@ namespace vcpkg::PlatformExpression
  {"openbsd", Identifier::openbsd},
  {"osx", Identifier::osx},
  {"uwp", Identifier::uwp},
  + {"xbox", Identifier::xbox},
  {"android", Identifier::android},
  {"emscripten", Identifier::emscripten},
  {"ios", Identifier::ios},
  @@ -519,7 +521,7 @@ namespace vcpkg::PlatformExpression
  // Point out in the diagnostic that they should add to the override list because that is
  // what most users should do, however it is also valid to update the built in identifiers to
  // recognize the name.
  - msg::println_error(msgUnrecognizedIdentifier, msg::value = expr.identifier);
  + msg::println_warning(msgUnrecognizedIdentifier, msg::value = expr.identifier);
  return false;
  case Identifier::x64: return true_if_exists_and_equal("VCPKG_TARGET_ARCHITECTURE", "x64");
  case Identifier::x86: return true_if_exists_and_equal("VCPKG_TARGET_ARCHITECTURE", "x86");
  @@ -541,6 +543,9 @@ namespace vcpkg::PlatformExpression
  case Identifier::osx: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "Darwin");
  case Identifier::uwp:
  return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "WindowsStore");
  + case Identifier::xbox:
  + // This identifier is intended to be enabled via VCPKG_DEP_INFO_OVERRIDE_VARS
  + return false;
  case Identifier::android: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "Android");
  case Identifier::emscripten:
  return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "Emscripten");
  