Platform guessing affects more than just Linux

[radarhere]

Despite

Pillow/setup.py

Line 347 in 5bff2f3

("disable-platform-guessing", None, "Disable platform guessing on Linux"),

the disable_platform_guessing setting also affects macOS and Cygwin.

Pillow/setup.py

Lines 553 to 566 in 5bff2f3

	if self.disable_platform_guessing:
	pass
	elif sys.platform == "cygwin":
	# pythonX.Y.dll.a is in the /usr/lib/pythonX.Y/config directory
	self.compiler.shared_lib_extension = ".dll.a"
	_add_directory(
	library_dirs,
	os.path.join(
	"/usr/lib", "python{}.{}".format(*sys.version_info), "config"
	),
	)
	elif sys.platform == "darwin":