depends: Explicitly specify clang search paths for darwin host

depends/hosts/darwin.mk

@@ -6,19 +6,25 @@ LD64_VERSION=530
 
 OSX_SDK=$(SDK_PATH)/Xcode-$(XCODE_VERSION)-$(XCODE_BUILD_ID)-extracted-SDK-with-libcxx-headers
 
+ifeq ($(strip $(FORCE_USE_SYSTEM_CLANG)),)
+clang_resource_dir=$(build_prefix)/lib/clang/$(native_cctools_clang_version)
+else
+clang_resource_dir=$(shell clang -print-resource-dir)
+endif
+
 # Flag explanations:
 #
 #     -mlinker-version
 #
 #         Ensures that modern linker features are enabled. See here for more
 #         details: https://github.com/bitcoin/bitcoin/pull/19407.
 #
-#     -B$(build_prefix)/bin
+#     -B"$(build_prefix)/bin"
 #
 #         Explicitly point to our binaries (e.g. cctools) so that they are
 #         ensured to be found and preferred over other possibilities.
 #
-#     -nostdinc++ -isystem $(OSX_SDK)/usr/include/c++/v1
+#     -stdlib=libc++ -nostdinc++ -Xclang -cxx-isystem"$(OSX_SDK)/usr/include/c++/v1"
 #
 #         Forces clang to use the libc++ headers from our SDK and completely
 #         forget about the libc++ headers from the standard directories
@@ -28,8 +34,42 @@ OSX_SDK=$(SDK_PATH)/Xcode-$(XCODE_VERSION)-$(XCODE_BUILD_ID)-extracted-SDK-with-
 #         https://reviews.llvm.org/D64089, we should use that instead. Read the
 #         differential summary there for more details.
 #
-darwin_CC=clang -target $(host) -mmacosx-version-min=$(OSX_MIN_VERSION) --sysroot $(OSX_SDK) -mlinker-version=$(LD64_VERSION) -B$(build_prefix)/bin
-darwin_CXX=clang++ -target $(host) -mmacosx-version-min=$(OSX_MIN_VERSION) --sysroot $(OSX_SDK) -stdlib=libc++ -mlinker-version=$(LD64_VERSION) -B$(build_prefix)/bin -nostdinc++ -isystem $(OSX_SDK)/usr/include/c++/v1
+#     -Xclang -*system"<path_a>" \
+#     -Xclang -*system"<path_b>" \
+#     -Xclang -*system"<path_c>" ...
+#
+#         Adds path_a, path_b, and path_c to the bottom of clang's list of
+#         include search paths. This is used to explicitly specify the list of
+#         system include search paths and its ordering, rather than rely on
+#         clang's autodetection routine. This routine has been shown to:
+#             1. Fail to pickup libc++ headers in $SYSROOT/usr/include/c++/v1
+#                when clang was built manually (see: https://github.com/bitcoin/bitcoin/pull/17919#issuecomment-656785034)
+#             2. Fail to pickup C headers in $SYSROOT/usr/include when
+#                C_INCLUDE_DIRS was specified at configure time (see: https://gist.github.com/dongcarl/5cdc6990b7599e8a5bf6d2a9c70e82f9)
+#
+#         Talking directly to cc1 with -Xclang here grants us access to specify
+#         more granular categories for these system include search paths, and we
+#         can use the correct categories that these search paths would have been
+#         placed in if the autodetection routine had worked correctly. (see:
+#         https://gist.github.com/dongcarl/5cdc6990b7599e8a5bf6d2a9c70e82f9#the-treatment)
+#
+#         Furthermore, it places these search paths after any "non-Xclang"
+#         specified search paths. This prevents any additional clang options or
+#         environment variables from coming after or in between these system
+#         include search paths, as that would be wrong in general but would also
+#         break #include_next's.
+#
+darwin_CC=clang -target $(host) -mmacosx-version-min=$(OSX_MIN_VERSION) \
+                -mlinker-version="$(LD64_VERSION)" -B"$(build_prefix)/bin" \
+                --sysroot "$(OSX_SDK)" \
+                -Xclang -internal-externc-isystem"$(clang_resource_dir)/include" \
+                -Xclang -internal-externc-isystem"$(OSX_SDK)/usr/include"
+darwin_CXX=clang++ -target $(host) -mmacosx-version-min=$(OSX_MIN_VERSION) \
+                   -mlinker-version="$(LD64_VERSION)" -B"$(build_prefix)/bin" \
+                   --sysroot "$(OSX_SDK)" \
+                   -stdlib=libc++ -nostdinc++ -Xclang -cxx-isystem"$(OSX_SDK)/usr/include/c++/v1" \
+                   -Xclang -internal-externc-isystem"$(clang_resource_dir)/include" \
+                   -Xclang -internal-externc-isystem"$(OSX_SDK)/usr/include"
 
 darwin_CFLAGS=-pipe
 darwin_CXXFLAGS=$(darwin_CFLAGS)