perhaps a better solution would be to adapt the GDBM_CFLAGS and GDBM_LIBS boilerplate

You mean creating NCURSES_CFLAGS and NCURSES_LIBS?

That would be CURSES_CFLAGS and CURSES_LIBS.

$ ./configure --help| grep -i curses
  CURSES_CFLAGS
              C compiler flags for CURSES, overriding pkg-config
  CURSES_LIBS linker flags for CURSES, overriding pkg-config

Yeah, I meant adapting the devguide's GDBM_CFLAGS and GDBM_LIBS recipe for CURSES_FLAGS and CURSES_LIBS but, as it appears the root causes were actually a caching issue and using a newer Apple SDK on an older system, that we shouldn't need to use the Homebrew ncurses going forward so this can probably safely be closed.