Tweaking order of defaults so that -shared is default across platforms unless explicitly overridden

src/rebar_port_compiler.erl

@@ -247,8 +247,8 @@ default_env() ->
      {"CXX", "g++"},
      {"CFLAGS", "-g -Wall -fPIC"},
      {"CXXFLAGS", "-g -Wall -fPIC"},
+     {"LDFLAGS", "-shared"},
      {"darwin", "LDFLAGS", "-bundle -flat_namespace -undefined suppress"},
-     {"linux", "LDFLAGS", "-shared"},
      {"DRIVER_CFLAGS", lists:concat([" -I", code:lib_dir(erl_interface, include),
                                      " -I", filename:join(erts_dir(), include),
                                      " "])},