VIM will automatically enable the TAB for a makefile, assuming you name it “makefile,” as opposed to “Makefile.” Not sure why VIM still doesn’t detect the type with a lower-uppercase difference, but such is life. (@Sedrik)
That aside, other alternative solutions are:
Filetype Binding (@ThorstenS @tungd):
autocmd FileType make setlocal noexpandtab
RealTime Switch (@ThorstenS):
Assuming the .vimrc configuration mentioned in the question, do:
:set noet (to switch from spaces to TAB)
and :set et (to switch back)