Add -isysroot to macOS native compile flags (#1203)

* 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/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.

* build: use -isysroot over --sysroot on macOS

Co-authored-by: Russell Yanofsky <[email protected]>
Co-authored-by: fanquake <[email protected]>
Co-authored-by: Prasanna Loganathar <[email protected]>

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:=$(shell xcrun -f clang) -isysroot$(shell xcrun --show-sdk-path)
+build_darwin_CXX:=$(shell xcrun -f clang++) -isysroot$(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)
@@ -10,8 +10,8 @@ build_darwin_SHA256SUM=shasum -a 256
 build_darwin_DOWNLOAD=curl --location --fail --connect-timeout $(DOWNLOAD_CONNECT_TIMEOUT) --retry $(DOWNLOAD_RETRIES) -o
 
 #darwin host on darwin builder. overrides darwin host preferences.
-darwin_CC=$(shell xcrun -f clang) -mmacosx-version-min=$(OSX_MIN_VERSION)
-darwin_CXX:=$(shell xcrun -f clang++) -mmacosx-version-min=$(OSX_MIN_VERSION) -stdlib=libc++
+darwin_CC=$(shell xcrun -f clang) -mmacosx-version-min=$(OSX_MIN_VERSION) -isysroot$(shell xcrun --show-sdk-path)
+darwin_CXX:=$(shell xcrun -f clang++) -mmacosx-version-min=$(OSX_MIN_VERSION) -stdlib=libc++ -isysroot$(shell xcrun --show-sdk-path)
 darwin_AR:=$(shell xcrun -f ar)
 darwin_RANLIB:=$(shell xcrun -f ranlib)
 darwin_STRIP:=$(shell xcrun -f strip)