The GNUmakefile.postamble file is an optional file you may include in your package to define additional rules that should be executed when making and/or installing the project. There is a template GNUmakefile.postamble file in the Makefile package that you can use as an example. Most of the rules are self explanatory. The before- rules define things that should happen before a process is executed (e.g. before-all for before compilation, before-install for before installation). The after- rules define things that should happen after a process is complete.
You can even define additional rules such as ones that a particular to your specific package or that are to be used by developers only.