Merge #17118: build: depends macOS: point --sysroot to SDK

a0daea459c [build] depends macOS: point --sysroot to SDK (Sjors Provoost)

Pull request description:

  Fixes errors like `fatal error: 'unistd.h' file not found` when building depends on macOS.

  Replaces #14352 (which doesn't work on Catalina).

ACKs for top commit:
  jonasschnelli:
    utACK a0daea459c

Tree-SHA512: 995b1e1e84e635b32d1d4038bc63730c94a7c318b7240f6d62825977e5c97fe52c5aa5a0f39070beb0df8271dd294b36d6b5cf7f09ad07494fb15d5bd4d77f68

depends/builders/darwin.mk

@@ -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) --sysroot $(shell xcrun --show-sdk-path)
+darwin_CXX:=$(shell xcrun -f clang++) -mmacosx-version-min=$(OSX_MIN_VERSION) -stdlib=libc++ --sysroot $(shell xcrun --show-sdk-path)
 darwin_AR:=$(shell xcrun -f ar)
 darwin_RANLIB:=$(shell xcrun -f ranlib)
 darwin_STRIP:=$(shell xcrun -f strip)