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

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 <[email protected]> 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 fanquake <[email protected]> pointed out in
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.

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)