Makefile editing slightly broken

I dunno what happened, I'm still using the same .vimrc I have for years, but lately there's
a glitch when I'm editing a Unix makefile.

When I start to edit the commands for a rule, it uses spaces instead of a tab to position
the command.  For makefiles, of course, this is wrong.  Vim even knows this, and highlights
the line in red. 

If I add lines after existing commands it gets the indentation right, perhaps because it copies the indentation of the previous line.

What should I look for?  What should I fix?

--

Kevin O'Gorman

#define QUESTION ((bb) || (!bb))   /* Shakespeare */

Please consider the environment before printing this email.

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