gdbm: avoid a warning on Darwin

This is just to minimize rebuilds. The actual bugfix doesn't seem to
hurry, as the function has been returning void until now, so if the int
returned isn't a meaningful value in some cases, nothing should happen
yet.

pkgs/development/libraries/gdbm/default.nix

···
 
 stdenv.mkDerivation rec {
   name = "gdbm-1.17";
+  # FIXME: remove on update to > 1.17
+  NIX_CFLAGS_COMPILE = if stdenv.cc.isClang then "-Wno-error=return-type" else null;
 
   src = fetchurl {
     url = "mirror://gnu/gdbm/${name}.tar.gz";