Added debugging rule to Makefile.

Added a debugging Makefile rule stolen from [1]. With this rule, you can
run make print-<VAR> to print out the contents of <VAR>. For example,

        make print-OBJS
        make print-PROTOOBJS
        make print-BINS

[1]: https://blog.melski.net/2010/11/30/makefile-hacks-print-the-value-of-any-variable/