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

 set(LINUX 1)
 set(UNIX 1)

 # Match multiarch library directory names.
 set(CMAKE_LIBRARY_ARCHITECTURE_REGEX "[a-z0-9_]+(-[a-z0-9_]+)?-linux-[a-z0-9_]*")
	set(LINUX 1)
	set(UNIX 1)

	# Match multiarch library directory names.
	set(CMAKE_LIBRARY_ARCHITECTURE_REGEX "[a-z0-9_]+(-[a-z0-9_]+)?-linux-[a-z0-9_]*")