Sorry, I don't understand what you want Automake to do. Right now, as far as I can tell, Automake does nothing with .POSIX. It's not mentioned in the manual nor, as far as I can grep, the code. Maybe that's the issue, and you want a leading .POSIX in your Makefile.am to be specially copied at the beginning of the output Makefile.in? (Or an option, or something.)
But ... have you tried to run a Makefile originating from Automake with .POSIX in effect? I feel doubtful that it will work, although I don't know it for a fact. -k
