depends: Add --sysroot option to mac os native compile flags

Catalina SDK clang stopped automatically searching the SDK include paths when
invoked without --sysroot:

https://github.com/bitcoin/bitcoin/pull/16367#issuecomment-594600985
https://github.com/Homebrew/homebrew-core/issues/45061

This hasn't been a problem for current native depends packages because are
passing their own --sysroot values, and hasn't been a problem for current host
packages because they use `darwin_` commands instead of `build_darwin_`
commands.  But the current `build_darwin_CC` and `build_darwin_CXX` commands
are still unnecessarily fragile, and incompatible with new native depends
packages added in https://github.com/bitcoin/bitcoin/pull/18677.

Cory Fields <cory-nospam-@coryfields.com> suggested in
https://github.com/bitcoin/bitcoin/pull/16367#issuecomment-595393546 switching
compiler from SDK clang to native clang (from $PATH) to avoid this problem.
This is easy and makes a certain amount of sense for building native packages,
as opposed to host packages. But fanquake <fanquake@gmail.com> pointed out in
https://github.com/bitcoin/bitcoin/pull/18677#discussion_r409934309 that it
would be inconsistent use switch to non-SDK compilers while still using other
SDK tools like ranlib and install_name_tool. So simplest, minimal fix seems to
be just adding the missing --sysroot option.

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