The Makefiles use $(DESTDIR) where it should use $(DESTDIR)$(PREFIX) instead. At least, that's the convention that is conventionally expected (*) (**):
(^) $(DESTDIR)$(PREFIX) or $(DESTDIR)/$(PREFIX) varies, but the latter seems safe in all cases (I gues?).
(*) https://people.freebsd.org/~rodrigc/doc/data/doc/en_USa.ISO8859-1/books/porters-handbook/porting-prefix.html
(**) https://www.gnu.org/software/make/manual/html_node/DESTDIR.html