Merge bitcoin#18743: depends: Add --sysroot option to mac os native compile flags

1e94a2b depends: Add --sysroot option to mac os native compile flags (Russell Yanofsky)

Pull request description:

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

  - 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#18677.

  Cory Fields (theuni) suggested in 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 Michael (fanquake) pointed out in bitcoin#18677 (comment) that it would be inconsistent to 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.

ACKs for top commit:
  ryanofsky:
    > ACK [1e94a2b](bitcoin@1e94a2b) - I think this change is ok, and I prefer it to the previous patch.
  fanquake:
    ACK 1e94a2b - I think this change is ok, and I prefer it to the previous patch. Thanks for the summary in the PR description. I played around with Xcode and the CLT; I think previously I didn't fully grok the slight differences between the two.

Tree-SHA512: 4d4bbb7f49acb76d934a872a15b4e14f36290b508cb9e728815f959767ec174bcfb6d2ca7dcd995cc550d86980d64d4247ea5ecfca2301f0953006e50744fdb4

Author: fanquake

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)