bpo-36146: Refactor setup.py (GH-12093)

* Rename globals to upper case to better distinguish if a variable is
  global or local:

  * Rename cross_compiling to CROSS_COMPILING
  * Rename host_platform to HOST_PLATFORM
  * Rename disabled_module_list to DISABLED_MODULE_LIST

* Add MS_WINDOWS, CYGWIN and MACOS constants.
* Use booleans: replace "return 0" with "return False"
  and replace "return 1" with "return True"