I suspect it will also need the /Q option, at least on the one with the wildcard, otherwise there will be a prompt. Though I'm not entirely clear on where this is run (nmake?) so maybe it's already suppressed.

The fact that I'm not familiar with this makefile suggests it isn't needed for normal Windows builds though, and if this is only meant for Cygwin/MinGW builds then I think assuming rf availability is fine.