Modules/Platform/watchOS-Initialize.cmake - third_party/github.com/Kitware/CMake - Git at Google

 include(Platform/Darwin-Initialize)

 if(NOT _CMAKE_OSX_SYSROOT_PATH MATCHES "/Watch(OS|Simulator)")
   message(FATAL_ERROR "${CMAKE_OSX_SYSROOT} is not an watchOS SDK")
 endif()
	include(Platform/Darwin-Initialize)

	if(NOT _CMAKE_OSX_SYSROOT_PATH MATCHES "/Watch(OS\|Simulator)")
	message(FATAL_ERROR "${CMAKE_OSX_SYSROOT} is not an watchOS SDK")
	endif()