GNU Make is a both very powerful and very popular make utility. It might even be called the de facto standard for make utilities. For more information see the GNU Make home page:
The cc65 development package is available as binaries for several host systems and can easily built for quite some additional systems. The very same is true for GNU Make so a cc65-based project coming with a GNU Make Makefile can easily be built by any cc65 developer no matter what host system is used.
Because of the strong alignment of the cc65 compiler with the ISO C standard it is very well feasible to compile a single C code base both with the cc65 compiler and other C compilers like for example GCC. GNU Make turns out to be very well suited to build projects for several target systems using multiple compilers as it isn't tied to any C compiler.