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?