I don't buy this argument. About 88% of the currently over 39k distributions in my (up-to-date) CPAN::Mini contain a Makefile.PL.
Update: To clarify: I'm not saying Makefiles are necessarily portable. The point is the solutions to the OP can be implemented in a portable way, but the way the above statement is worded makes it seem like they can't. That's the problem with a four-word post; perhaps clarification would have been useful.