I am doubtful about blithely defining .POSIX unconditionally. I feel sure that will break existing Makefiles. I don't think we should do that.
Detecting .POSIX in an existing Makefile.am and moving it to the front sounds desirable, since that is clearly what the developer intended. Another (not mutually exclusive) possibility is to add an Automake option (say, "make-posix") that outputs the .POSIX line. --thanks, karl.
