Are makefiles intended to be portable across platforms?

[Angel Tsankov]

Am I taking a bad path if I try to write one makefile to manage (e.g., build 
& clean) a project on a couple of platforms?
I'm asking this question, 'casue makefiles seem not to be very portable, 
right?
For example, to define a clean target one might have to write:

clean: ; if exist OutputFiles rmdir /S /Q OutputFiles

or

clean: ; if exist OutputFiles rmdir OutputFiles -R

depending on the platform that make is run on.

Regards,
Angel Tsankov