The 'make-generated-sources.sh' script does exactly what its name says. It creates source files. Take a closer look at DerivedSources.make: all of the (non-phony) targets (start of a line, before a colon) are .c and .h files. It never even invokes a compiler or linker, and doesn't even reference the vast majority of the code, so it certainly isn't a makefile for the entire thing.

Also, you can run 'make-generated-sources.sh' on pretty much any Linux or UNIX (probably including Cygwin).