I wrote "the GNU Make Book has the code ready for you to study." I'm currently traveling, without access to my books, but a quick online search at the contents

I wrote "the GNU Make Book has the code ready for you to study." I'm currently traveling, without access to my books, but a quick online search at the contents of this book https://nostarch.com/download/GNU_Make_dTOC.pdf https://nostarch.com/download/GNU_Make_dTOC.pdf shows that p.82 has the code for "Rebuilding When a File's Checksum Changes". The GNU Make Book by John Graham-Cumming, No Starch Press, April 2015, 256 pp., ISBN-13: 978-1-59327-649-2