GNU Make: "Makefile:29: *** Missing separator"

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Issue #17 new
asmyers created an issue

When I try to build the book I get an error Makefile: 29: *** missing separator. This seems to be related to the -= on line 29:

images-svg -= $(images-dot:dot=svg)

I'm not a make expert but I can't find any documentation that describes a -= operator. I've tried this with GNU make 4.0 and 4.1. Is this a BSD make specific feature?