[CMake] determine 32 vs 64 bit linux

[Karl Merkley]

I need to install a library into a directory whose name depends on the machine type.   For example

if (WIN32)
  if (CMAKE_CL_64)
      set(ARCH_DIR "win32")
  else()
      set(ARCH_DIR "win64")
  endif()

elseif (UNIX)
  if (????)
      set(ARCH_DIR "linux32")
  else()
     set(ARCH_DIR "linux64")
endif()

Is there a better way to do this?   The win64 version seems to work correctly.   What variable do I check for 32 vs 64 bit linux?

  Thanks,
        Karl