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

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

bitcoin/bitcoin#16367 (comment)
Homebrew/homebrew-core#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
bitcoin/bitcoin#18677.

Cory Fields <[email protected]> suggested in
bitcoin/bitcoin#16367 (comment)
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
<[email protected]> pointed out in
bitcoin/bitcoin#18677 (comment) 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.
```

Backport of core [[bitcoin/bitcoin#18743 | PR18743]].
Note that it doesn't fix any real life problem for now.

Test Plan: Run the OSX Gitian build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7522

Author: ryanofsky

depends/builders/darwin.mk

@@ -1,5 +1,5 @@
-build_darwin_CC:=$(shell xcrun -f clang)
-build_darwin_CXX:=$(shell xcrun -f clang++)
+build_darwin_CC:=$(shell xcrun -f clang) --sysroot $(shell xcrun --show-sdk-path)
+build_darwin_CXX:=$(shell xcrun -f clang++) --sysroot $(shell xcrun --show-sdk-path)
 build_darwin_AR:=$(shell xcrun -f ar)
 build_darwin_RANLIB:=$(shell xcrun -f ranlib)
 build_darwin_STRIP:=$(shell xcrun -f strip)