depends: fix zlib build on osx

zlib is sneaky and expects ar to be libtool on darwin.

depends/packages/zlib.mk

@@ -7,8 +7,10 @@ $(package)_sha256_hash=c3e5e9fdd5004dcb542feda5ee4f0ff0744628baf8ed2dd5d66f8ca11
 define $(package)_set_vars
 $(package)_build_opts= CC="$($(package)_cc)"
 $(package)_build_opts+=CFLAGS="$($(package)_cflags) $($(package)_cppflags) -fPIC"
-$(package)_build_opts+=AR="$($(package)_ar)"
 $(package)_build_opts+=RANLIB="$($(package)_ranlib)"
+$(package)_build_opts+=AR="$($(package)_ar)"
+$(package)_build_opts_darwin+=AR="$($(package)_libtool)"
+$(package)_build_opts_darwin+=ARFLAGS="-o"
 endef
 
 define $(package)_config_cmds