Makefile customization is easy.  Simply copy the default "Makefile" into
a file named "makefile" or possibly "GNUMakefile" or "BSDMakefile" (depending
on what version of make you are running) and then apply your edits to the
copy.  The "make" program looks for its makefile under name (GNU|BSD)Makefile,
makefile, and Makefile, in that order.  So if you supply one of the earlier
names, the one included with the Fossil distribution will not be used.

You could also make a copy of the file into "local.mk" and then do
"make -f local.mk".