https://bugzilla.redhat.com/show_bug.cgi?id=1794912
--- Comment #14 from Artur Iwicki fedora@svgames.pl --- Taking a brief peek at "rpmbuild --showrc", %{set_build_flags} is called automatically when using %{configure}, %{cmake} or %{meson} - I guess that because of this, most Fedora packages have no need to call it explicitly in %build.
Still, yeah, it would be a good idea to add that somewhere to the packaging docs - I think the C/C++ guidelines would be the most appropriate place.