feat(cc toolchain): support external_include_paths on macos

The external_include_paths feature enables specifying include paths locally with -I and with -isystem when the project is included as an external repo.

This makes it possible to set -Werror against your own headers for development without propagating this to consumers of your libraries.

For some reason this was only previously enabled on linux. I do not see a reason for not also enabling it on macos.

Closes bazelbuild#21949.

PiperOrigin-RevId: 634394452
Change-Id: I0df91b2a2c9b4a5bd52b22e64f99ea0745cb9759

tools/cpp/unix_cc_toolchain_config.bzl

@@ -1515,6 +1515,7 @@ def _impl(ctx):
             default_link_flags_feature,
             user_link_flags_feature,
             default_link_libs_feature,
+            external_include_paths_feature,
             fdo_optimize_feature,
             dbg_feature,
             opt_feature,