Perhaps you're misunderstanding the man page. The synopsis section typically provides an overview of what is needed to use that function. It gives you anything you need to #define (_GNU_SOURCE in this case) and any files you need to #include (string.h in this case). It also gives you the prototype, so you know how to call it, and know what parameters are being referred to in the "Description" section. Defining the _GNU_SOURCE macro enables the GNU extensions, like strverscmp. Also, notice that the man page says "See feature_test_macros(7)". Whenever you see some_topic(N), that is telling you to look at the man page for some_topic, in section N. You do that by typing "man N some_topic". In this case "man 7 feature_test_macros" would provide you with
this info, further detailing what _GNU_SOURCE does.