Set copt for internal tools like make and pkgconfig

[mering]

Please add some way to pass compiler flags for building the rules_foreign_cc internal tools.

We want to build our own code base with -Werror but we don't have a way to disable warnings for those external tool we don't have control over.

[keith]

rules_foreign_cc could definitely expose some starlark flags for this type of thing. I think using --host_copt does apply to some of these now, but that might be too wide for what you want. You could also patch in the flags until this is resolved:

rules_foreign_cc/foreign_cc/built_tools/make_build.bzl

Line 101 in be14cca

"CFLAGS": _join_flags_list(ctx.workspace_name, non_sysroot_cflags),

[mering]

@keith Thanks for the code hint. Although I explicitly want to enable -Werror also for --host_copt to build all of our own tools with -Werror as well.

[jsharpe]

PRs would be welcome in this area but this isn't something that I'll have bandwidth to get to doing anytime in the near future.