Merge "Remove setting _USING_LIBCXX"

my_cxx_ldlibs :=

ifneq ($(filter $(my_cxx_stl),libc++ libc++_static),)

    my_cflags += -D_USING_LIBCXX

    ifeq ($($(my_prefix)OS),darwin)

        # libc++'s headers are annotated with availability macros that indicate

        # which version of Mac OS was the first to ship with a libc++ feature