How to set noexpandtab for a specific file

I have the following two lines in my .vimrc:
:set tabstop=4 softtabstop=4 shiftwidth=4 expandtab
set modeline

On the first line of a file called automake.am, I have
# vim: set noexpandtab:

When I open automake.am and enter a tab I spaces instead of tabs.

How do I set noexpandtab for automake.am automatically, so I do
not forget to enter it every time I edit that file ?

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